Discover how your eyes work, why some people need glasses, how a prism splits white light into a rainbow, and why the sky is blue but sunsets are red.Janiye ki हमारी eyes kaise kaam करती हैं, logo को specs (चश्मा) क्यों लगता है, prism white light को rainbow colors mein kaise split करता है, और sky blue पर sunset red क्यों होता है।
🌐 Language / भाषा:
Remember from before:
In Chapter 9, we learned that light bends (refracts) when it passes from one transparent medium to another.Chapter 9 mein हमने सीखा था ki light जब ek transparent medium से दूसरे transparent medium mein जाती है, तो वह bend (refract) होती है।
We used the lens formula ($1/v - 1/u = 1/f$) and found that convex lenses converge light while concave lenses diverge it.हमने lens formula ($1/v - 1/u = 1/f$) का उसे किया था और देखा था ki convex lenses light ko converge (इकट्ठा) karte hain, jabki concave lenses light ko diverge (फैलाते) karte hain।
Power of a lens ($P = 1/f$) is measured in dioptres (D) — positive for convex, negative for concave.Lens ki power ($P = 1/f$) को dioptres (D) mein measure किया jata है — convex lens के liye ये positive और concave lens के liye negative होती है।
1. The Human Eye1. The Human Eye (मानव नेत्र)
Section Goal: After this section you will be able to label the main parts of the human eye, explain how the eye forms images on the retina, and describe the power of accommodation.Section Goal: Is section के बाद आप human eye के main parts को label कर सकेंगे, ये explain कर सकेंगे ki eye retina पर image kaise बनती है, और power of accommodation को describe कर सकेंगे।
📷 The Camera Analogy📷 The Camera Analogy (कैमरा का उदाहरण)
Your eye works exactly like a camera. The cornea is the front glass of the camera lens that does most of the bending of light. The iris is the aperture ring that opens wide in the dark and shrinks in bright light. The crystalline lens does the fine-focus adjustments. And the retina at the back is the camera film (or digital sensor) where the image actually forms!आपकी eye बिल्कुल ek camera ki तरह kaam करती है। cornea camera lens का front glass है जो light को sabse ज़्यादा bend करता है। iris aperture ring ki तरह है जो andhere mein बड़ा और bright light mein छोटा हो jata है। crystalline lens fine-focus adjustments करता है। और back mein present retina camera film (या digital sensor) ki तरह है jahan image actually बनती है!
The human eye is one of the most valuable and sensitive sense organs. It allows us to see the wonderful world and colours around us [NCERT p.164]. Let's look at how this remarkable organ is built.Human eye हमारे sabse valuable और sensitive sense organs mein से ek है। ये हमें हमारे aas-paas ki सुंदर दुनिया और रंगों को dekhne के काबिल बनती है [NCERT p.164]। आइए dekhein ki ये shandar organ kaise बना है।
Figure 10.1 — 3D medical anatomy render of the human eye showing corneal refraction and internal structures.Figure 10.1 — Human eye का 3D medical anatomy render जो corneal refraction और internal structures को दिखाता है।
Figure 10.1 — Structure of the Human Eye. Light enters through the cornea, passes through the pupil, gets focused by the lens, and forms an inverted image on the retina. Adapted from NCERT Figure 10.1.Figure 10.1 — Human eye की structure। Light, cornea से enter करती है, pupil से pass होती है, lens द्वारा focus होती है, और retina पर inverted image बनाती है। NCERT Figure 10.1 पर आधारित।
Parts of the Human Eye
PartPart (भाग)
What it doesWhat it does (कार्य)
Cornea (कॉर्निया)Cornea (कॉर्निया / cornea)
Thin, transparent front surface. Does most of the refraction (bending) of incoming light [NCERT p.164].Eyeball ki patli और transparent front surface। Incoming light का sabsay zyada refraction (bending) isi surface पर होता है [NCERT p.164]।
Iris (आइरिस / परितारिका)Iris (आइरिस / iris)
Coloured muscular diaphragm behind the cornea. Controls the size of the pupil [NCERT p.164].Cornea के पीछे present coloured muscular diaphragm। ये pupil के size को control करता है [NCERT p.164]।
Pupil (पुतली)Pupil (pupil)
The dark circular opening. Regulates the amount of light entering the eye [NCERT p.164]. Opens wide in dim light, shrinks in bright light.आँख के बीच का black circular opening। ये eye mein enter करने wali light ki quantity को regulate और control करता है [NCERT p.164]। Dim light mein ये फैल jata है, bright light mein सिकुड़ jata है।
Biconvex lens made of fibrous, jelly-like material. Fine-focuses light onto retina. Its curvature can be adjusted by ciliary muscles [NCERT p.164].Fibrous और jelly-like material से बना biconvex lens। ये light को retina पर fine-focus करता है। Iski curvature ciliary muscles द्वारा adjust हो सकती है [NCERT p.164]।
Hold the lens and change its shape (curvature). Contract to make lens thicker, relax to make lens thinner [NCERT p.165].Lens को hold करती हैं और iski curvature (shape) को change करती हैं। Contract hokar lens को thick और relax hokar lens को thin बनती हैं [NCERT p.165]।
Retina (दृष्टिपटल)Retina (retina)
Delicate membrane at the back of the eye. Contains light-sensitive cells: rods (for dim-light vision) and cones (for colour vision) [NCERT p.164].Eye के back part ki delicate membrane (screen)। Inme light-sensitive cells होते हैं: rods (dim-light के liye) और cones (colour vision के liye) [NCERT p.164]।
Carries electrical signals from the retina to the brain for image processing [NCERT p.164].Retina पर बने electrical signals को brain तक carry करती है taaki image को process किया jaa सके [NCERT p.164]।
Blind Spot (अंध बिन्दु)Blind Spot (अंध बिन्दु)
The point where the optic nerve exits. Has no light-sensitive cells — an image falling here cannot be seen [NCERT p.164].Aisa point jahan से optic nerve exit होती है। Yahan कोई light-sensitive cells nahi होते — isliye yahan बनने wali image दिखाई nahi देती [NCERT p.164]।
Power of AccommodationPower of Accommodation (power of accommodation)
The ability of the eye lens to adjust its focal length is called the power of accommodation[NCERT p.165]. Ciliary muscles adjust the curvature of the eye lens to change its focal length:Eye lens ki अपनी focal length को adjust करने ki ability को power of accommodation (power of accommodation) kehte हैं [NCERT p.165]। Ciliary muscles lens ki curvature को adjust karke iski focal length को बदलती हैं:
Looking at a distant object (far away): Ciliary muscles relax. Lens becomes thin. Focal length increases to its maximum. Parallel light rays focus perfectly on the retina [NCERT p.165].Looking at a distant object (दूर की चीजें): Ciliary muscles relax (शिथिल) हो जाती हैं। Lens thin (पतला) हो जाता है। Focal length maximum हो जाती है। Parallel light rays retina par perfectly focus hoti hain [NCERT p.165]।
Looking at a nearby object: Ciliary muscles contract. Lens becomes thick (more curved). Focal length decreases. Diverging light from the near object is brought to focus on the retina [NCERT p.165].Looking at a nearby object (पास की चीजें): Ciliary muscles contract (सिकुड़) जाती हैं। Lens thick (मोटा) हो जाता है। Focal length decrease हो जाती है। Diverging light rays retina par focus hoti hain [NCERT p.165]।
Key Concept: The near point (least distance of distinct vision) for a normal adult eye is 25 cm. The far point is at infinity [NCERT p.166]. If you hold a book closer than 25 cm, your ciliary muscles strain and you cannot focus clearly.Key Concept: Normal adult eye ke liye near point (स्पष्ट दृष्टि की न्यूनतम दूरी) 25 cm hoti hai, aur far point infinity (अनंत) par hota hai [NCERT p.166]। अगर आप किसी book को 25 cm से पास rakhenge, तो ciliary muscles पर strain padega और आप clear nahi देख payenge।
Move the slider to change the object distance. Watch how the ciliary muscles change the lens shape and focal length to keep the image on the retina!Object ki distance बदलने के liye slider move करें। Dekhein kaise ciliary muscles lens ki shape और focal length को change karke image को retina पर बनाए रखती हैं!
200 cm (far)
Normal Vision (Far Object): Ciliary muscles are relaxed. The lens is thin, with a long focal length. Parallel rays from the distant object converge exactly on the retina.
⚡ Checkpoint 1
What is meant by the power of accommodation of the eye? [NCERT p.166, In-text Q1]Eye की power of accommodation से क्या मतलब है? [NCERT p.166, In-text Q1]
Eye Structure: Cornea does most refraction → iris/pupil regulate light → crystalline lens fine-focuses → image forms (inverted) on retina → optic nerve sends signals to brain.Eye Structure: Cornea refracts light → iris/pupil regulate light quantity → crystalline lens fine-focuses → retina पर inverted image बनती है → optic nerve signals को brain तक भेजती है।
Accommodation: Ciliary muscles change lens shape. Contract (thick lens, short f) for near objects. Relax (thin lens, long f) for far objects.Accommodation: Ciliary muscles lens ki shape बदलती हैं। Near objects के liye contract (thick lens, short f) और far objects के liye relax (thin lens, long f) होती हैं।
Limits: Near point = 25 cm. Far point = infinity for a normal eye.Limits: Normal eye ke liye near point = 25 cm aur far point = infinity (अनंत) hota hai।
2. Defects of Vision and their Correction2. Defects of Vision and their Correction (vision defect और उनका संशोधन)
Section Goal: After this section you will be able to identify myopia (निकट-दृष्टि दोष), hypermetropia (दीर्घ-दृष्टि दोष) and presbyopia (जरा-दूरदृष्टिता), explain their causes, and calculate the corrective lens power needed.Section Goal: Is section ke baad aap myopia (निकट-vision defect), hypermetropia (दीर्घ-vision defect) aur presbyopia (जरा-दूरदृष्टिता) ko identify kar sakenge, inke causes explain kar sakenge, aur corrective lens power ko calculate kar sakenge।
🔍 The Projector Focus Analogy🔍 The Projector Focus Analogy (प्रोजेक्टर फोकस का उदाहरण)
Imagine a movie projector. If the screen is too close, the image is blurry beyond it. If the screen is too far, the image is blurry before it reaches. Your retina is the screen. In myopia, the "projector" (eye) focuses the image BEFORE the screen (retina) — you see blurry far objects. In hypermetropia, the image tries to form BEHIND the screen — you see blurry near objects. Corrective lenses act like adjusting the projector to hit the screen perfectly!Ek movie projector के baare mein सोचें। अगर screen bohot पास है, तो image blurry हो jayegi। अगर screen bohot दूर है, तो image पहले ही blur हो jayegi। आपकी retina screen है। Myopia mein, eye image को screen (retina) के पहले focus करती है — isliye दूर ki cheezein blurry दिखती हैं। Hypermetropia mein, image screen के पीछे बनती है — isliye पास ki cheezein blurry दिखती हैं। Corrective lenses projector को adjust करने ki तरह हैं taaki focus screen पर perfectly ban सके!
Sometimes the eye loses its ability to focus clearly. There are three common defects of vision [NCERT p.166]:कभी-कभी eye ki clear focus करने ki capability कम हो जाती है। Vision के 3 common defects होते हैं [NCERT p.166]:
A person with myopia can see nearby objects clearly but cannot see distant objects distinctly [NCERT p.166].Myopia से peedit person पास ki cheezon को तो clearly देख सकता है, पर दूर ki cheezon को distinctly nahi देख paata [NCERT p.166]।
Causes: (i) Excessive curvature of the eye lens, OR (ii) elongation of the eyeball [NCERT p.166].Causes (कारण): (i) Eye lens ki excessive curvature (यानी लेंस का बहुत अधिक वक्र होना), OR (ii) eyeball ka elongated (लंबा) ho jana [NCERT p.166]।
What happens: The image of a distant object forms in front of the retina (not on it) [NCERT p.166].What happens: Distant object ki image retina के आगे (front) बनती है, retina पर nahi [NCERT p.166]।
Correction: Use a concave lens of appropriate power. It diverges the light rays slightly before they enter the eye, shifting the image back onto the retina [NCERT p.166].Correction (संशोधन): Sutable power के concave lens (अवतल लेंस) ka use kiya jata hai। Ye light rays ko enter karne se pehle thoda diverge kar deta hai, jisse image peeche shift hokar retina par banti hai [NCERT p.166]।
Formula: Focal length of the corrective lens $f = -d$ (where $d$ is the far point distance). Power $P = 1/f$ (in meters) [NCERT p.166].Formula: Corrective lens ki focal length $f = -d$ (jahan $d$ far point distance है)। Power $P = 1/f$ (meters mein) [NCERT p.166]।
A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly [NCERT p.167].Hypermetropia से peedit person दूर ki cheezon को clearly देख सकता है, पर पास ki cheezon को distinctly nahi देख paata [NCERT p.167]।
Causes: (i) Focal length of the eye lens is too long, OR (ii) the eyeball is too short [NCERT p.167].Causes (कारण): (i) Eye lens ki focal length का bahut बड़ा hona, OR (ii) eyeball का छोटा hona [NCERT p.167]।
What happens: The image of a nearby object forms behind the retina [NCERT p.167].What happens: Nearby object ki image retina के पीछे (behind) बनती है [NCERT p.167]।
Correction: Use a convex lens of appropriate power. It converges light rays slightly more, bringing the image forward onto the retina [NCERT p.167].Correction (संशोधन): Suitable power के convex lens (उत्तल लेंस) ka use kiya jata hai। Ye light rays ko thoda converge karta hai, jisse image aage shift hokar retina par banti hai [NCERT p.167]।
This is an age-related condition. As a person grows older, the ciliary muscles gradually weaken and the crystalline lens loses its flexibility [NCERT p.168]. The near point gradually recedes away (becomes more than 25 cm).ये उम्र के sath hone wali condition है। जैसे-जैसे इंसान ki उम्र बढ़ती है, ciliary muscles gradually kamzor हो जाती हैं और crystalline lens अपनी flexibility खो देता है [NCERT p.168]। Isse near point दूर चला jata है (25 cm से ज़्यादा हो jata है)।
Correction: Use bifocal lenses — the upper part is a concave lens for seeing distant objects, and the lower part is a convex lens for reading [NCERT p.168].Correction (संशोधन):bifocal lenses (द्विफोक्सी lenses) ka use hota hai — jisme upper part concave lens (door dekhne ke liye) aur lower part convex lens (reading ke liye) hota hai [NCERT p.168]।
Sometimes a person may have both myopia and hypermetropia. Such a person also needs bifocal lenses [NCERT p.168].कभी-कभी किसी person को myopia और hypermetropia dono हो सकते हैं। Aise person को भी bifocal lenses ki zarurat होती है [NCERT p.168]।
Defect
Cannot See
Image FormsImage Forms (छवि कहाँ बनती है)
Cause
Corrective LensCorrective Lens (संशोधन लेंस)
Myopia
Far objectsDistant objects (दूर की वस्तुएं)
Before retinaBefore retina (रेटिना से पहले)
Lens too curved / eyeball too longLens का बहुत ज़्यादा curved होना / eyeball का बहुत लंबा (too long) होना
Concave (−ve power)Concave lens (अवतल लेंस)
Hypermetropia
Near objectsNearby objects (पास की वस्तुएं)
Behind retinaBehind retina (रेटिना के पीछे)
Lens too flat / eyeball too shortLens का बहुत ज़्यादा flat होना / eyeball का बहुत छोटा (too short) होना
Convex (+ve power)Convex lens (उत्तल लेंस)
Presbyopia
Near objects (age-related)Nearby objects (उम्र-संबंधी)
Behind retina
Weak ciliary muscles / rigid lensCiliary muscles का कमज़ोर (weak) होना / lens का rigid होना
👓 Try-It Lab 2: 3D Vision Defect & Correction Lab👁️ Try-It Lab 2: 3D Vision Defect & Correction Lab (vision defect और लेंस सुधारक सैंडबॉक्स)
Rotate the 3D eyeball model by dragging. Select a defect state to see where the parallel light rays focus inside the elongated or short eyeball. Toggle the corrective lens to fix the focus on the retina!Drag karke 3D eyeball model को rotate करें। Defect state को select karke dekhein ki parallel rays kahan focus होती हैं। Corrective lens toggle karke focus को retina पर wapas shift करें!
Initializing 3D WebGL...
Normal Vision: Parallel light rays focus exactly on the retina at the back.
🔢 Lens Power Calculator🔢 Lens Power Calculator (लेंस क्षमता कैलकुलेटर)
Enter the far point (for myopia) or near point (for hypermetropia) and click Calculate.Far point (myopia के liye) या near point (hypermetropia के liye) enter करें और Calculate button पर click करें।
⚡ Checkpoint 2
A student sitting in the last row has difficulty reading the blackboard. What defect of vision does the student likely have, and how can it be corrected? [NCERT p.168, In-text Q4]Last row mein baithe student को blackboard पढ़ने mein problem होती है। उसे kya vision defect हो सकता है, और isse kaise correct किया ja सकता है? [NCERT p.168, In-text Q4]
📝 Section Summary
Myopia: Cannot see far. Image before retina. Fix: concave lens ($P = 1/f$, where $f = -d$ far point distance).Myopia: दूर ki cheezein nahi dikhtin। Image retina के आगे बनती है। Fix: concave lens ($P = 1/f$, jahan $f = -d$ far point distance)।
Hypermetropia: Cannot see near. Image behind retina. Fix: convex lens.Hypermetropia: Paas ki cheezein nahi dikhtin। Image retina ke peeche banti hai। Fix: convex lens (उत्तल लेंस)।
Presbyopia: Age-related loss of accommodation. Near point recedes. Fix: bifocal lens.Presbyopia: Age के sath accommodation का loss। Near point recedes। Fix: bifocal lens।
3. Refraction of Light through a Prism3. Refraction of Light through a Prism (प्रिज़्म से प्रकाश का अपवर्तन)
Section Goal: After this section you will be able to trace the path of a light ray through a triangular glass prism and identify the angle of incidence, angle of emergence, and angle of deviation.Section Goal: Is section के बाद आप ek triangular glass prism से light ray के path को trace कर सकेंगे और angle of incidence, angle of emergence, और angle of deviation को identify कर सकेंगे।
🏔️ The Mountain Road Analogy🏔️ The Mountain Road Analogy (पहाड़ी सड़क का उदाहरण)
Imagine driving on a straight road. Suddenly you reach a mountain (the prism). You have to take a turn at the entry slope and another turn at the exit slope. The total change in your direction from the original straight road is like the angle of deviation. The steeper the mountain's slopes (prism angle), the sharper you have to turn!Maan lijiye आप ek straight road पर drive कर rahe हैं। Achanak raste mein ek pahad (prism) आता है। आपको entry slope पर ek turn lena padta है और exit slope पर dusra turn। आपकी direction mein जो total change aaya है, उसे angle of deviation ki तरह समझें। Pahad ki slopes jitni teez (prism angle) hongi, आपको utna ही sharp turn lena padega!
A prism is a transparent optical element with flat, polished surfaces that refract light [NCERT p.168]. It has two triangular bases and three rectangular lateral surfaces. The angle of the prism (A) is the angle between the two refracting surfaces [NCERT p.169].Prism ek transparent optical element है jiski flat, polished refracting surfaces होती हैं [NCERT p.168]। Iske पास do triangular bases और teen rectangular lateral surfaces होती हैं। Do refracting surfaces के बीच के angle को angle of the prism (A) kehte हैं [NCERT p.169]।
Figure 10.2 — Refraction of light through a triangular glass prism. The light bends toward the base at both surfaces. δ is the angle of deviation. Adapted from NCERT Figure 10.5.
When a ray of light enters a prism, it bends toward the base at the first surface, travels through the prism, and bends toward the base again at the second surface [NCERT p.169]. The angle between the original direction of the incident ray and the direction of the emergent ray is called the angle of deviation (δ)[NCERT p.169].जब light ray prism mein enter करती है, तो ये first surface पर base ki taraf bend होती है, prism के अंदर travel करती है, और second surface पर wapas base ki taraf bend हो कर निकलता है [NCERT p.169]। Incident ray ki original direction और emergent ray ki direction के बीच के total angle को angle of deviation (δ) (विचलन कोण) kehte hain [NCERT p.169]।
⚡ Checkpoint 3
When light passes through a glass prism, which direction does it bend? [NCERT p.169]जब light किसी glass prism से गुज़रती है, तो यह किस direction में bend होती है? [NCERT p.169]
📝 Section Summary
Prism: A triangular glass body with angle A between its two refracting surfaces.Prism: Ek triangular glass body jiske do refracting surfaces के बीच angle A होता है।
Bending Direction: Light bends toward the base at both surfaces.Bending Direction: Light rays dono surfaces पर base ki taraf bend होती हैं।
Angle of Deviation (δ): The total angle between the incident ray direction and emergent ray direction.Angle of Deviation (δ): Incident ray ki original direction और emergent ray ki direction के बीच का total angle।
4. Dispersion of White Light by a Glass Prism4. Dispersion of White Light by a Glass Prism (काँच के प्रिज़्म द्वारा श्वेत प्रकाश का वर्ण-विक्षेपण)
Section Goal: After this section you will be able to explain what dispersion is, name the 7 colours of the visible spectrum (VIBGYOR), describe Newton's recombination experiment, and explain how rainbows form.Section Goal: Is section के बाद आप dispersion kya है ये explain कर सकेंगे, spectrum के सातों रंगों (VIBGYOR) के नाम बता सकेंगे, Newton के recombination experiment को describe कर सकेंगे, और rainbow kaise बनता है ये explain कर सकेंगे।
🎵 The Music Band Analogy🎵 The Music Band Analogy (म्यूजिक बैंड का उदाहरण)
Imagine a music band marching in step. As long as they march on a smooth road (air), they stay together — that's white light. When they hit a muddy patch (glass prism), the slower instruments (shorter wavelengths like violet) get stuck and slow down more, while the faster ones (longer wavelengths like red) speed through more easily. They spread out! That spreading is dispersion. If there's another mud patch tilted the other way (second inverted prism), they all sync up again — recombination back to white light!Ek music band के baare mein सोचें जो kadam से kadam मिला कर chal raha है। जब तक वह smooth road (air) पर chalte हैं, वह sath रहते हैं — ये white light है। जब वह ek muddy patch (glass prism) पर पहुंचते हैं, तो slow चलने wale instruments (short wavelengths जैसे violet) slow हो जाते हैं, jabki fast चलने wale (long wavelengths जैसे red) आसानी से nikal जाते हैं। वह फैल जाते हैं! ये फैलना ही dispersion है। अगर दूसरा mud patch inverted (ulta prism) लगा दिया jaye, तो वह सब फिर से sync हो जाते हैं — yaani white light mein recombination हो jata है!
White light (sunlight) splits into a band of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (remembered as VIBGYOR) [NCERT p.169]. This band of colours is called a spectrum.White light (sunlight) 7 colors के band mein split हो जाती है: Violet, Indigo, Blue, Green, Yellow, Orange, Red (जिसे VIBGYOR kehte हैं) [NCERT p.169]। रंगों के is band को spectrum kehte हैं।
The splitting of white light into its component colours is called dispersion (वर्ण-विक्षेपण)[NCERT p.169]. It happens because different colours have different wavelengths, and each wavelength bends by a different amount when passing through the prism.White light का अपने component colours mein split hona dispersion (वर्ण-विक्षेपण) kehlata है [NCERT p.169]। Aisa isliye होता है kyunki different colours ki different wavelengths होती हैं, और हर wavelength prism से गुज़रते waqt alag angle पर bend होती है।
Violet (shortest wavelength) bends the most[NCERT p.169].Violet (sabse छोटी wavelength) sabse zyada bend होती है [NCERT p.169]।
Red (longest wavelength) bends the least[NCERT p.169].Red (sabse लंबी wavelength) sabse kam bend होती है [NCERT p.169]।
Newton's Experiment: RecombinationNewton's Experiment: Recombination (न्यूटन का प्रयोग: पुनर्combination)
Isaac Newton placed a second identical prism inverted (upside-down) in the path of the dispersed spectrum. The second prism recombined all seven colours back into white light [NCERT p.170]. This proved conclusively that white light is actually composed of seven colours mixed together.Isaac Newton ne dispersed spectrum के path mein ek दूसरा identical prism inverted (ulta) rakh दिया। दूसरे prism ne सातों रंगों को फिर से white light mein recombine कर दिया [NCERT p.170]। Isse conclusively prove हो गया ki white light actually सातों रंगों से मिलकर बनी है।
RainbowRainbow (इंद्रधनुष)
A rainbow is a natural spectrum seen in the sky after rain [NCERT p.170]. Tiny water droplets in the atmosphere act as small prisms. Sunlight entering a water droplet undergoes refraction, internal reflection, and dispersion — producing the beautiful arc of VIBGYOR colours [NCERT p.170].Rainbow ek natural spectrum है जो baarish के बाद sky mein दिखता है [NCERT p.170]। Atmosphere mein suspended पानी ki छोटी बूंदें tiny prisms ki तरह kaam करती हैं। Sunlight जब इन droplets mein enter करती है, तो refraction, internal reflection, और dispersion से गुज़र कर VIBGYOR रंगों का beautiful arc बनती है [NCERT p.170]।
Watch white light split into VIBGYOR through the first prism. Toggle the second inverted prism to recombine the colours back into white light (Newton's experiment)!First prism से white light को VIBGYOR mein split होते hue dekhein। सातों रंगों को wapas white light mein recombine करने के liye दूसरा inverted (ulta) prism toggle करें (Newton का experiment)!
Dispersion: White light is composed of 7 colours (VIBGYOR). A prism splits them because violet (shortest wavelength) bends the most and red (longest wavelength) bends the least [NCERT p.169].
⚡ Checkpoint 4
Which colour of light bends the most when passing through a prism, and why? [NCERT p.169]Prism से गुज़रते समय light का कौनसा colour सबसे ज़्यादा bend होता है, और क्यों? [NCERT p.169]
📝 Section Summary
Dispersion: Splitting of white light into 7 colours (VIBGYOR) by a prism.Dispersion: Prism द्वारा white light का 7 colours (VIBGYOR) mein split hona।
Reason: Different colours have different wavelengths → different bending angles.Reason: Different wavelengths के karan different colours का refractive index और bending angle alag-alag होता है।
Newton's Proof: A second inverted prism recombines VIBGYOR back into white light.Newton's Proof: Ek दूसरा inverted prism spectum के सातों रंगों को wapas white light mein recombine कर देता है।
Rainbow: Natural dispersion by water droplets (refraction + internal reflection + dispersion).Rainbow: Water droplets द्वारा natural dispersion (refraction + internal reflection + dispersion)।
Section Goal: After this section you will be able to explain why stars twinkle, why planets don't twinkle, and why the sun is visible slightly before actual sunrise and after actual sunset.Section Goal: Is section के बाद आप explain कर सकेंगे ki tare क्यों टिमटिमते हैं, planets क्यों nahi टिमटिमते, और sun actual sunrise से पहले और actual sunset के बाद तक दिखता है।
🌊 The Swimming Pool Bottom Analogy🌊 The Swimming Pool Bottom Analogy (स्विमिंग पूल का उदाहरण)
Look at a coin at the bottom of a swimming pool. The water is slightly moving, and the coin seems to shimmer and shift position. That's because the water's surface is constantly changing, bending the light randomly. Stars twinkle for the same reason — Earth's atmosphere is like a giant, turbulent swimming pool of air above us. The starlight keeps getting bent in slightly different directions as atmospheric conditions change!Ek swimming pool के bottom mein रखे coin को dekhein। पानी के थोड़े hilne पर coin shimmer और shift होता hua दिखता है। Aisa isliye है kyunki पानी ki surface lagatar बदलती रहती है और light को randomly bend करती है। Stars भी isi wajah से twinkle करते हैं — Earth ki atmosphere हमारे upar air का ek बड़ा, ashant pool है। जैसे-जैसे atmospheric conditions बदलती हैं, starlight lagatar alag-alag directions mein bend होती रहती है!
The Earth's atmosphere is made of layers of air with varying optical density. The density of air decreases with altitude [NCERT p.171]. Since light bends when it passes from one density to another, starlight is continuously refracted as it passes through different atmospheric layers [NCERT p.171].Earth ka atmosphere alag-alag optical density wale air layers se bana hai। Altitude (ऊंचाई) ke sath air ki density kam hoti jaati hai [NCERT p.171]। Chunki light jag ek density से दूसरी density mein जाती है तो bend होती है, isliye starlight atmosphere के layers से गुज़रते waqt continuously refract होती रहती है [NCERT p.171]।
Twinkling of StarsTwinkling of Stars (तारों का टिमटिमाना)
Stars are very far away and appear as point sources of light [NCERT p.171]. As starlight enters the Earth's atmosphere, it passes through layers of air with constantly changing temperature and density. This causes the apparent position of the star to change slightly and continuously. The path of the light keeps fluctuating, so the amount of light reaching our eyes changes — making the star appear to twinkle [NCERT p.171].Stars bohot दूर हैं और light के point sources ki तरह दिखते हैं [NCERT p.171]। Starlight जब Earth के atmosphere mein enter करती है, तो ये constant change hone wale temperature और density के air layers से गुज़रती है। Is wajah से star ki apparent position lagatar थोड़ी बदलती रहती है। Light का path fluctuate होता रहता है, jisse आँख mein enter करने wali light ki quantity बदलती है — और star twinkle करता hua दिखता है [NCERT p.171]।
Planets are much closer to Earth than stars. They appear as extended sources (not point sources) — like a collection of many point sources of light [NCERT p.172]. The fluctuations caused by atmospheric refraction from individual points average out. Overall, the brightness remains steady, so planets don't twinkle [NCERT p.172].Planets, stars ke मुकाबले Earth ke bohot paas hain। Wo point sources nahi balki extended sources (बड़े स्रोत) ki tarah dikhte hain — jaise bohot saare point sources ka collection ho [NCERT p.172]। Individual points से hone wali fluctuations average out हो जाती हैं। Isliye total brightness steady रहती है और planets twinkle nahi करते [NCERT p.172]।
The Sun is visible about 2 minutes before it actually rises above the horizon, and about 2 minutes after it actually sets below the horizon [NCERT p.172].Sun actual sunrise (क्षितिज से ऊपर आना) se lagbhag 2 minutes pehle aur actual sunset ke lagbhag 2 minutes baad tak dikhta rehta hai [NCERT p.172]।
This happens because atmospheric refraction bends the sunlight toward the observer even when the Sun is still below the geometric horizon. The light from the Sun follows a curved path through the atmosphere (from less dense to more dense layers), making the Sun appear slightly above its actual position [NCERT p.172].Aisa atmospheric refraction के karan होता है, जो sunlight को bend karke observer तक pahunchata है jabki sun horizon के नीचे ही होता है। Sun से आने wali light atmosphere से गुज़रते waqt curve path follow करती है, jisse sun actual position से थोड़ा upar दिखता है [NCERT p.172]।
Quick Fact: Because of atmospheric refraction, each day is actually about 4 minutes longer than it would be if the Earth had no atmosphere (2 minutes extra at sunrise + 2 minutes at sunset) [NCERT p.172].
⚡ Checkpoint 5
Why do stars twinkle, but planets do not? [NCERT p.173, Ex Q8-Q9]Stars क्यों twinkle करते हैं, पर planets नहीं, ऐसा क्यों? [NCERT p.173, Ex Q8-Q9]
📝 Section Summary
Atmospheric Refraction: Light bends through layers of atmosphere with varying density.Atmospheric Refraction: Atmosphere के different density layers से passing light का refractive bending।
Stars Twinkle: Point sources → apparent position fluctuates → brightness varies.Stars Twinkle: Point sources → apparent position fluctuate होती है → brightness change होती है।
Planets Don't Twinkle: Extended sources → individual fluctuations average out.Planets Don't Twinkle: Extended sources → individual point fluctuations average out हो जाती हैं।
Extra Daylight: Sun visible ~2 min before sunrise and ~2 min after sunset due to atmospheric bending.Extra Daylight: Atmospheric bending के karan Sun actual sunrise से ~2 min पहले और sunset के ~2 min बाद तक visible रहता है।
6. Scattering of Light6. Scattering of Light (scattering of light)
Section Goal: After this section you will be able to explain the Tyndall effect, Rayleigh scattering, why the sky is blue, why sunsets are red, why clouds are white, and why danger signals are red.Section Goal: Is section के बाद आप Tyndall effect, Rayleigh scattering, sky के blue hone का reason, sunset/sunrise पर sun के red hone का reason, और danger signals के red hone का reason explain कर सकेंगे।
🔦 The Torch-in-a-Dusty-Room Analogy🔦 The Torch-in-a-Dusty-Room Analogy (धूल भरे कमरे का उदाहरण)
Shine a torch in a clean room — you can't see the beam. Now shake a dusty blanket and shine again — you see the light beam clearly! The dust particles scatter the light sideways toward your eyes. That's the Tyndall effect. Now imagine that the dust only scatters blue light well and lets red pass through. If you look at the torch FROM THE SIDE, you see blue. If you look AT the torch FROM FAR AWAY through the dust, you see the remaining red light. That's exactly why the sky is blue (side view) and sunsets are red (looking through a thick layer of atmosphere)!Ek साफ़ kamre mein torch jalayein — आपको beam nahi dikhegi। अब ek dhul bhare blanket को jhadkar torch jalayein — आपको light beam clearly dikhegi! Dust particles light को sideways scatter करते हैं, jisse वह हमारी आँखों तक पहुंचती है। Yahi Tyndall effect है। अब सोचें ki dust सिर्फ blue light को scatter करती है और red को guzarne देती है। अगर आप side से dekhenge, तो blue dikhega। और अगर bohot दूर से dekhenge, तो सिर्फ बची hui red light dikhega। Yahi reason है ki sky blue दिखता है और sunset/sunrise पर sun red दिखता है!
Tyndall Effect (टिंडल प्रभाव)
The scattering of light by colloidal particles (tiny particles suspended in a medium) is called the Tyndall effect[NCERT p.173].Colloidal particles (tiny suspended particles) द्वारा light का scatter hona Tyndall effect (टिंडल प्रभाव) kehlata hai [NCERT p.173]।
A beam of sunlight becomes visible when it enters a dusty room through a window [NCERT p.173].Window से जब sunbeam किसी dhul bhare kamre mein enter करती है, तो uska path visible हो jata है [NCERT p.173]।
The path of sunlight becomes visible through the forest canopy as particles of mist scatter the light [NCERT p.173].Forest canopy (घने जंगलों के वितान) se guzarte waqt mist particles light ko scatter karte hain, jisse path visible ho jata hai [NCERT p.173]।
The blue colour of smoke rising from an incense stick (agarbatti) is due to scattering by fine smoke particles [NCERT p.173].अगरबत्ती से nikalne wale dhuen का blue colour fine smoke particles द्वारा light के scattering के karan होता है [NCERT p.173]।
Rayleigh Scattering (रेले प्रकीर्णन)
According to Lord Rayleigh, the amount of scattering is inversely proportional to the fourth power of the wavelength of light [NCERT p.173]:Lord Rayleigh के अनुसार, light की scattering का amount उसकी wavelength की fourth power के inversely proportional होता है [NCERT p.173]:
Scattering ∝ 1 / λ4
This means shorter wavelengths (like blue and violet) scatter much more than longer wavelengths (like red and orange). Since blue light has roughly half the wavelength of red, it scatters about 24 = 16 times more than red!Iska matlab है ki short wavelengths (जैसे blue और violet) long wavelengths (जैसे red और orange) के comparison mein bohot ज़्यादा scatter होती हैं। Chunki blue light ki wavelength red ki lagbhag aadhi होती है, isliye ये red से lagbhag 24 = 16 times ज़्यादा scatter होती है!
Why is the Sky Blue? (आसमान नीला क्यों है?)
Fine particles in the atmosphere (molecules of N2 and O2) are much smaller than the wavelength of visible light. These particles scatter shorter wavelengths (blue) much more effectively than longer wavelengths (red) [NCERT p.173]. The scattered blue light reaches our eyes from all directions, making the sky appear blue.Atmosphere के fine particles (N2 और O2 molecules) visible light ki wavelength से bohot छोटे होते हैं। ये particles short wavelengths (blue) को long wavelengths (red) के comparison mein ज़्यादा effectively scatter करते हैं [NCERT p.173]। ये scattered blue light हमारी आँखों तक सभी directions से पहुंचती है, jisse sky blue दिखता है।
Why not violet? Our eyes are more sensitive to blue than violet, and some violet gets absorbed in the upper atmosphere. So the sky appears blue, not violet [NCERT p.173].Violet क्यों nahi? हमारी आँखें violet के comparison mein blue के liye ज़्यादा sensitive हैं, और कुछ violet upper atmosphere mein absorb हो jata है। Isliye sky blue दिखता है, violet nahi [NCERT p.173]।
Why Does the Sky Appear Dark from Space?
In outer space, there is no atmosphere to scatter sunlight. Without scattering, the sky appears dark and black, and you can see stars even during the daytime [NCERT p.173].Outer space mein कोई atmosphere nahi होता। बिना scattering के, sky बिल्कुल black (dark) दिखता है, और आप दिन mein भी stars देख सकते हैं [NCERT p.173]।
Why Do Clouds Appear White?
Clouds contain relatively large water droplets. These droplets are much bigger than the wavelength of visible light. Large particles scatter all wavelengths equally. Since all colours are scattered equally and reach our eyes together, clouds appear white [NCERT p.174].Clouds mein पानी ki बड़ी droplets होते हैं जो visible light ki wavelength से kafi बड़ी होती हैं। बड़ा size hone के karan ये सभी wavelengths (रंगों) को equally scatter करती हैं। सभी रंग barabar scatter hokar जब हमारी आँख तक पहुंचते हैं, तो clouds white दिखते हैं [NCERT p.174]।
Why Does the Sun Appear Reddish at Sunrise and Sunset?
At sunrise and sunset, sunlight has to travel through a much thicker layer of atmosphere to reach our eyes. During this long journey, most of the blue and shorter-wavelength light gets scattered away by atmospheric particles. Only the longer-wavelength light (red and orange) manages to reach our eyes without being scattered much [NCERT p.174]. That's why the Sun and the sky near the horizon look reddish.Sunrise और sunset के समय, sunlight को हमारी आँख तक pahunchne के liye atmosphere ki मोटी layer से travel करना padta है। Is लंबे safar mein, blue और short wavelengths atmospheric particles द्वारा scatter hokar बाहर nikal जाती हैं। सिर्फ long wavelength light (red और orange) ही बिना ज़्यादा scatter hue हम तक पहुंच paati है [NCERT p.174]। Isliye sun और horizon के पास का sky reddish दिखता है।
Why are Danger Signals Red?
Red light has the longest wavelength among visible colours and therefore scatters the least. It can travel through fog, smoke, and mist farther than any other colour without being scattered away [NCERT p.174]. That's why danger signals, brake lights, and traffic stop lights are red — they're visible from the greatest distance.Red light ki wavelength visible spectrum mein sabse lambi hoti hai, isliye ye sabse kam scatter hoti hai। Ye fog (कोहरे), dhuen (smoke) aur mist mein se bina scatter hue doosre rangon ke comparison mein door tak jaa sakti hai [NCERT p.174]। Yahi reason है ki danger signals, brake lights और traffic stop lights red होते हैं — taaki ये sabse दूर से भी dikh sakein।
Move the sun from noon to the horizon. Watch the sky colour change from blue to orange/red as the light path through the atmosphere increases!Sun ko noon (dopahar) se horizon (क्षितिज) tak move karein। Dekhein kaise atmosphere mein light path badhne par sky ka colour blue se orange/red ho jata hai!
Noon (overhead)
Noon Sky: The sun is high overhead. Sunlight travels through a thin layer of atmosphere. Blue light is scattered effectively in all directions by fine atmospheric molecules, making the sky appear blue[NCERT p.173].
⚡ Checkpoint 6
Why does the Sun appear reddish early in the morning? [NCERT p.174, Ex Q10]सुबह के समय Sun हमें reddish क्यों दिखाई देता है? [NCERT p.174, Ex Q10]
📝 Section Summary
Tyndall Effect: Scattering of light by colloidal/fine particles (dusty room, smoke, forest canopy).Tyndall Effect: Colloidal/fine particles द्वारा light का scatter hona (dusty room, smoke, forest canopy)।
Rayleigh Scattering: Scattering ∝ $1/\lambda^4$. Blue scatters ~16× more than red.Rayleigh Scattering: Scattering ∝ $1/\lambda^4$। Blue light red के comparison mein ~16× ज़्यादा scatter होती है।
Blue Sky: Blue scattered most by fine N2/O2 molecules.Blue Sky: Atmospheric N2/O2 molecules द्वारा blue light sabsay ज़्यादा scatter होती है।
Red Sunrise/Sunset: Long atmospheric path scatters away blue; only red/orange reaches eye.Red Sunrise/Sunset: लंबा atmospheric path blue को scatter कर देता है; सिर्फ red/orange आँख तक पहुंचता है।
White Clouds: Large water droplets scatter all colours equally.White Clouds: बड़ी water droplets सभी रंगों को equally scatter करती हैं, jisse ये सफेद दिखते हैं।
Dark Sky in Space: No atmosphere = no scattering.Dark Sky in Space: Space mein कोई atmosphere nahi होता, isliye wahan black (dark) दिखता है।
NCERT Textbook Solutions
In-text Questions
Q1. What is meant by power of accommodation of the eye?
Answer: The ability of the eye lens to adjust its focal length (by changing its shape through ciliary muscles) is called the power of accommodation. When ciliary muscles contract, the lens becomes thicker (shorter focal length) to focus nearby objects. When they relax, the lens becomes thinner (longer focal length) to focus distant objects [NCERT p.165].
Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?Q2. Ek myopic person 1.2 m से दूर ki objects को distinctly nahi देख पाता। Proper vision को restore करने के liye kis type का corrective lens उसे किया jana चाहिए?
Answer: The person has myopia (near-sightedness). A concave lens should be used. Since the far point is 1.2 m, the focal length of the corrective lens $f = -1.2$ m. Power $P = 1/f = 1/(-1.2) =$ $-0.83$ D (approximately) [NCERT p.166].Answer: Is person को myopia (निकट-vision defect) hai। Isko correct karne ke liye concave lens (अवतल लेंस) use kiya jana chahiye। Chunki far point 1.2 m है, तो corrective lens ki focal length $f = -1.2$ m hogi। Power $P = 1/f = 1/(-1.2) =$ $-0.83$ D (approximately) [NCERT p.166]।
Q3. What is the far point and near point of the human eye with normal vision?Q3. Normal vision वाले human eye के लिए near point और far point क्या होते हैं?
Answer: For a person with normal vision: • Far point = Infinity (∞) — the farthest point up to which the eye can see clearly. • Near point = 25 cm — the closest point at which the eye can see clearly (also called least distance of distinct vision) [NCERT p.166].Answer: Normal vision wale person के liye: • Far point (दूर बिंदु) = Infinity (अनंत - ∞) — wo maximum distance jahan tak eye clear dekh sakti hai। • Near point (निकट बिंदु) = 25 cm — वह minimum distance jahan पर rakhi object को eye बिना stress के clear देख सकती है [NCERT p.166]।
Q4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect of vision in this case? How can it be corrected?Q4. Last bench पर baithe ek student को blackboard पढ़ने mein problem होती है। Is case mein vision defect kya हो सकता है, और iska correction kaise किया jayega?
Answer: The student has myopia (near-sightedness). He can see nearby objects (like his notebook) clearly but cannot see distant objects (like the blackboard). This can be corrected by using spectacles with a concave lens of appropriate power. The concave lens diverges the incoming light slightly before it enters the eye, so that the image of the distant blackboard shifts back onto the retina [NCERT p.166].Answer: Student को myopia (निकट-vision defect) hai। Wo paas ki objects (jaise notebook) ko clear dekh sakta hai par door ki objects (blackboard) ko nahi dekh paata। Is defect को correct करने के liye suitable power का concave lens (अवतल lens) spectacles mein use kiya jayega। Concave lens parallel light rays ko enter karne se pehle thoda diverge kar deta hai, jisse blackboard ki image shift hokar retina par banti hai [NCERT p.166]।
Exercise QuestionsExercise Questions (अभ्यास के प्रश्न)
Q1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to: (a) presbyopia (b) accommodation (c) near-sightedness (d) far-sightednessQ1. Eye lens ki focal length को adjust karke human eye different distances पर objects को focus कर सकती है। ये kiski wajah से होता है: (a) presbyopia (b) accommodation (c) near-sightedness (d) far-sightedness
Answer: (b) accommodation The ability of the eye lens to adjust its focal length to focus objects at different distances is called the power of accommodation [NCERT p.165].Answer: (b) accommodation (समंजन क्षमता) Different distances पर rakhi objects को focus करने के liye lens ki focal length adjust करने ki capability को power of accommodation kehte हैं [NCERT p.165]।
Q2. The human eye forms the image of an object at its: (a) cornea (b) iris (c) pupil (d) retinaQ2. Human eye किसी object की image कहाँ बनाती है: (a) cornea (b) iris (c) pupil (d) retina
Answer: (d) retina The retina acts as the screen of the eye where the image is formed [NCERT p.164].Answer: (d) retina (दृष्टिपटल) Retina eye ki screen ki तरह kaam करता है jahan actually image बनती है [NCERT p.164]।
Q3. The least distance of distinct vision for a young adult with normal vision is about: (a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 mQ3. Normal vision वाले young adult के लिए least distance of distinct vision लगभग कितनी होती है: (a) 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m
Answer: (c) 25 cm The least distance of distinct vision (near point) for a young adult with normal vision is approximately 25 cm [NCERT p.166].Answer: (c) 25 cm Normal vision wale young adult ke liye near point (स्पष्ट दृष्टि की न्यूनतम दूरी) lagbhag 25 cm hota hai [NCERT p.166]।
Q4. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to enable him to read clearly a book held at 25 cm?Q4. Diagram banakar dikhayein ki hypermetropia को kaise correct किया jata है। Ek hypermetropic eye का near point 1 m है। 25 cm पर rakhi book को clear पढ़ने के liye kis power के lens ki zarurat hogi?
Answer: Hypermetropia is corrected using a convex lens.
Calculation: The person wants to read a book at $u = -25$ cm (normal near point). But his near point is 1 m, so the image must form at $v = -100$ cm (virtual, same side). Using the lens formula: $1/v - 1/u = 1/f$ $1/(-100) - 1/(-25) = 1/f$ $-1/100 + 1/25 = 1/f$ $(-1 + 4)/100 = 1/f$ $3/100 = 1/f$ $f = 100/3 = +33.33$ cm = $+0.333$ m Power $P = 1/f = 1/0.333 =$ $+3$ D[NCERT p.167].Answer: Hypermetropia को convex lens (उत्तल लेंस) ka use karke correct kiya jata hai।
Calculation: Person 25 cm distance पर book पढ़ना chahta है, तो object distance $u = -25$ cm (normal near point)। But uska near point 1 m (100 cm) है, तो image $v = -100$ cm पर banni चाहिए (virtual image, same side)। Lens formula उसे करने पर: $1/v - 1/u = 1/f$ $1/(-100) - 1/(-25) = 1/f$ $-1/100 + 1/25 = 1/f$ $(-1 + 4)/100 = 1/f$ $3/100 = 1/f$ $f = +100/3$ cm = $+33.33$ cm = $+0.333$ m Power $P = 1/f = 1/0.333 =$ $+3$ D[NCERT p.167]।
Q5. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the corrective lens required to enable him to see very distant objects clearly?Q5. Ek myopic person का far point आँख के samne 80 cm है। Bahut दूर ki objects को clear dekhne के liye corrective lens ki nature और power kya hogi?
Answer: The person has myopia. Far point = 80 cm = 0.8 m. A concave lens is needed. The concave lens must form a virtual image of a distant object (at infinity) at the far point of the eye. For distant objects: $u = -\infty$, $v = -80$ cm = $-0.8$ m Using the lens formula: $1/v - 1/u = 1/f$ $1/(-0.8) - 1/(-\infty) = 1/f$ $-1/0.8 - 0 = 1/f$ $f = -0.8$ m Power $P = 1/f = 1/(-0.8) =$ $-1.25$ D Nature: Concave (diverging) lens with negative power [NCERT p.166].Answer: Person को myopia है। Far point = 80 cm = 0.8 m. Ek concave lens (अवतल lens) ki zarurat hogi। Concave lens ko infinity par rakhi door ki object ki virtual image far point (80 cm) par banani hogi। Distant objects के liye: $u = -\infty$, $v = -80$ cm = $-0.8$ m Lens formula से: $1/v - 1/u = 1/f$ $1/(-0.8) - 1/(-\infty) = 1/f$ $-1/0.8 - 0 = 1/f$ $f = -0.8$ m Power $P = 1/f = 1/(-0.8) =$ $-1.25$ D Nature: Negative power वाला concave (diverging) lens [NCERT p.166]।
Q6. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?Q6. Normal eye 25 cm से पास रखी objects को clearly क्यों नहीं देख पाती?
Answer: When an object is placed closer than 25 cm, the ciliary muscles cannot contract enough to make the lens sufficiently thick (curved) to reduce the focal length further. The image would form behind the retina, resulting in a blurry view. Therefore, 25 cm is the minimum distance at which a normal eye can see objects clearly without straining [NCERT p.166].Answer: जब किसी object को 25 cm से ज़्यादा पास रखा jata है, तो ciliary muscles lens को और ज़्यादा thick (curved) करने के liye contract nahi हो paati हैं, jisse focal length और ज़्यादा कम nahi हो सकती। Is wajah से light rays retina के पीछे focus होती हैं और image blurry दिखती है। Isliye, normal eye के बिना strain के clear dekhne के liye minimum distance 25 cm होती है [NCERT p.166]।
Q7. What happens to the image distance in the eye when we increase the distance of an object from the eye?Q7. जब हम eye से object की distance बढ़ाते हैं, तो eye के अंदर image distance पर क्या प्रभाव पड़ता है?
Answer: The image distance in the eye stays approximately the same. The image always forms on the retina (which is at a fixed distance from the lens). When the object distance increases, the ciliary muscles relax, the lens becomes thinner, and its focal length increases to ensure the image still forms on the retina [NCERT p.165].Answer: Eye के अंदर image distance lagbhag same रहती है। Image हमेशा retina पर ही बनती है (जो lens से fixed distance पर है)। जब object distance बढ़ती है, तो ciliary muscles relax हो जाती हैं, jisse lens पतला (thin) हो jata है और iski focal length बढ़ जाती है taaki image wapas retina पर ही focus हो सके [NCERT p.165]।
Q8. Why do stars twinkle?Q8. Stars क्यों twinkle करते हैं?
Answer: Stars are very distant and appear as point sources. Their light passes through different layers of the atmosphere with varying density and temperature. This causes the light's path to refract continuously and the apparent position of the star to change slightly. As the atmospheric conditions fluctuate, the amount of starlight entering our eyes varies, making stars appear to twinkle [NCERT p.171].Answer: Stars bohot दूर हैं और point sources ki तरह behave करते हैं। Unse आने wali light atmosphere के alag-alag layers (jinki temperature और density lagatar बदलती रहती है) से गुज़रती है। Is wajah से starlight का continuous refraction होता है और star ki apparent position थोड़ी बदलती रहती है। Atmosphere के fluctuation के karan आँख तक pahunchne wali starlight ki quantity बदलती रहती है, jisse stars टिमटिमते hue दिखते हैं [NCERT p.171]।
Q9. Explain why the planets do not twinkle.Q9. Explain करें ki planets क्यों nahi टिमटिमते।
Answer: Planets are much closer to Earth compared to stars. They appear as extended sources of light (not point sources). An extended source is essentially a collection of many point sources. The total variation in the amount of light from all these individual points averages out. As a result, the brightness of a planet remains steady and it does not appear to twinkle [NCERT p.172].Answer: Planets, stars के comparison mein Earth के kafi पास हैं। वह point sources के bajay extended sources (बड़े source) ki तरह दिखते हैं। Ek extended source को हम bohot सारे point sources का collection maan सकते हैं। इन individual points से आने wali light ki fluctuations आपस mein average out (neutralize) हो जाती हैं। Is wajah से planet ki overall brightness steady बनी रहती है और वह twinkle nahi करते [NCERT p.172]।
Q10. Why does the Sun appear reddish early in the morning?Q10. सुबह के समय Sun हमें reddish क्यों दिखाई देता है?
Answer: At sunrise (and sunset), the Sun is near the horizon. Its light must travel through a much thicker (longer) layer of the atmosphere to reach our eyes. During this long path, the shorter wavelengths (blue, violet) are scattered away in all directions by fine atmospheric particles (Rayleigh scattering: scattering ∝ 1/λ4). Only the longer wavelengths (red, orange) survive this journey and reach our eyes, making the Sun appear reddish [NCERT p.174].Answer: Sunrise (और sunset) के समय Sun horizon के पास होता है। Uske light को हमारी आँखों तक pahunchne के liye atmosphere ki kafi मोटी और लंबी layer से travel करना padta है। Is लंबे raste mein, short wavelengths (blue, violet) fine particles द्वारा charo taraf scatter (Rayleigh scattering $\propto 1/\lambda^4$) हो जाती हैं। सिर्फ लंबी wavelength wali light (red, orange) ही बिना ज़्यादा scatter hue हम तक पहुंच paati है, jisse Sun reddish दिखता है [NCERT p.174]।
Q11. Why does the sky appear dark instead of blue to an astronaut?Q11. Astronauts को sky blue के बजाय dark/black क्यों दिखाई देता है?
Answer: In outer space, there is no atmosphere. Since there are no atmospheric particles to scatter sunlight, no scattered light reaches the astronaut's eyes from different directions. The sky therefore appears dark and black instead of blue [NCERT p.173].Answer: Outer space mein कोई atmosphere nahi होता है। Chunki wahan sunlight को scatter करने के liye particles ही nahi होते, isliye किसी भी direction से scatter hokar light astronauts ki आँख तक nahi पहुंचती। Yahi karan है ki astronauts को sky blue के bajay black (dark) दिखता है [NCERT p.173]।
Solved CBSE Previous Year Questions (PYQs)
1 Mark Questions (Very Short Answer)
Q1. Name the type of lens used to correct (i) myopia, (ii) hypermetropia. [CBSE 2020]Q1. Un lenses के type बताएं जो (i) myopia, (ii) hypermetropia को correct करने के liye उसे किए जाते हैं। [CBSE 2020]
Answer: (i) Myopia is corrected using a concave lens (diverging lens). (ii) Hypermetropia is corrected using a convex lens (converging lens).Answer: (i) Myopia को correct करने के liye concave lens (अवतल लेंस) use hota hai। (ii) Hypermetropia को correct करने के liye convex lens (उत्तल लेंस) use hota hai।
Q2. What is the cause of dispersion of white light by a glass prism? [CBSE 2019]Q2. Glass prism dwara white light ke dispersion (वर्ण-विक्षेपण) ka kya cause hai? [CBSE 2019]
Answer: White light is made of seven colours with different wavelengths. Each wavelength bends by a different amount when passing through the glass prism (different refractive index for each colour). Violet (shortest λ) bends the most, red (longest λ) bends the least. This differential bending splits white light into its component colours.Answer: White light 7 colors से मिलकर बनी है jinki wavelengths different होती हैं। Glass prism से pass hone पर हर रंग ki wavelength alag refractive index के karan alag amount पर bend होती है। Violet (shortest $\lambda$) sabse ज़्यादा bend होता है और red (longest $\lambda$) sabse कम bend होता है। Is differential bending के karan white light अपने constituent colors mein split हो जाती है।
Q3. Why does the sky appear blue on a clear day? [CBSE 2022]Q3. एकदम साफ़ मौसम (clear day) में sky हमें blue क्यों दिखाई देता है? [CBSE 2022]
Answer: Fine molecules (N2, O2) in the atmosphere scatter shorter wavelengths of sunlight much more than longer wavelengths (Rayleigh scattering ∝ 1/λ4). Blue light, having a shorter wavelength, is scattered abundantly in all directions. This scattered blue light reaches our eyes from all parts of the sky, making it appear blue.Answer: Atmosphere के fine molecules ($N_2$, $O_2$) sunlight ki short wavelengths को long wavelengths के comparison mein kafi ज़्यादा scatter करते हैं (Rayleigh scattering $\propto 1/\lambda^4$)। Blue light ki wavelength छोटी hone के karan ये charo taraf bohot ज़्यादा scatter हो जाती है। ये scattered blue light जब हमारी आँखों तक पहुंचती है, तो sky blue दिखता है।
Q4. State the function of the iris in the human eye. [CBSE 2021]Q4. Human eye mein iris ka function (कार्य) kya hai? [CBSE 2021]
Answer: The iris is a coloured muscular diaphragm that controls the size of the pupil. In bright light, it contracts to make the pupil smaller (reducing light entry). In dim light, it expands to make the pupil larger (allowing more light to enter).Answer: Iris ek coloured muscular diaphragm है जो pupil के size को control करता है। Bright light mein, ये contract hokar pupil को छोटा कर देता है (jisse कम light enter kare)। Dim light mein, ये expand hokar pupil को बड़ा कर देता है (jisse ज़्यादा light enter कर सके)।
2 Marks Questions (Short Answer I)
Q5. Explain with the help of a diagram, how the defect of myopia can be corrected by using a suitable lens. [CBSE 2023]Q5. Diagram ki help se explain karein ki myopia (निकट-vision defect) ko suitable lens use karke kaise correct kiya jata hai। [CBSE 2023]
Answer: In myopia, the image of distant objects forms in front of the retina (either due to elongated eyeball or excessive lens curvature). A concave lens of appropriate power is placed before the eye. The concave lens diverges the incoming parallel rays slightly, so that after refraction by the eye lens, the image shifts back and forms on the retina.
The focal length of the concave lens equals the negative of the far point distance. For example, if far point = 1.5 m, then $f = -1.5$ m and $P = -0.67$ D.Answer: Myopia mein, दूर ki object ki image retina के आगे (front) बनती है (eyeball के लंबे hone या lens curvature के ज़्यादा hone से)। Ek suitable power का concave lens (अवतल लेंस) aankh ke samne lagaya jata hai। Concave lens incident parallel rays ko thoda diverge kar deta hai, jisse eye lens dwara refraction ke baad image wapas retina par focus ho jaati hai।
Corrective lens ki focal length far point distance के negative के barabar होती है। For example, अगर far point = 1.5 m है, तो $f = -1.5$ m और $P = -0.67$ D hoga।
Q6. What is meant by the 'power of accommodation' of the eye? How does it help in clear vision? [CBSE 2020]Q6. Eye की power of accommodation से क्या मतलब है? यह clear vision में कैसे मदद करती है? [CBSE 2020]
Answer: The power of accommodation is the ability of the eye lens to change its focal length by adjusting its curvature through ciliary muscles. • For distant objects: ciliary muscles relax → lens becomes thin → focal length increases → parallel rays focus on retina. • For nearby objects: ciliary muscles contract → lens becomes thick → focal length decreases → diverging rays focus on retina. This ensures objects at various distances are seen clearly.Answer: Power of accommodation eye lens ki वह ability है jiske threw ciliary muscles lens ki curvature और shape को change karke uski focal length को adjust करती हैं। • Distant objects (दूर की चीजें) के liye: ciliary muscles relax होती हैं → lens thin हो jata है → focal length बढ़ जाती है → parallel rays retina पर focus होती हैं। • Nearby objects (पास की चीजें) के liye: ciliary muscles contract होती हैं → lens thick हो jata है → focal length कम हो जाती है → diverging rays retina पर focus होती हैं। Is तरह different distances पर rakhi objects clear दिखती हैं।
3 Marks Questions (Short Answer II)
Q7. (a) Why do stars twinkle? (b) Why do planets not twinkle? (c) If we viewed the stars from outer space, would they appear to twinkle? [CBSE 2022]Q7. (a) Stars क्यों twinkle करते हैं? (b) Planets क्यों twinkle नहीं करते? (c) अगर stars को outer space से देखा जाए, तो क्या वे twinkle करेंगे? [CBSE 2022]
Answer: (a) Stars twinkle because they are very far away and appear as point sources. The light from stars passes through layers of atmosphere with varying density and temperature, causing continuous refraction. The apparent position of the star fluctuates slightly, and the amount of light reaching our eye varies — producing the twinkling effect.
(b) Planets are much closer and appear as extended sources (tiny discs, not points). They are collections of many point sources. Individual fluctuations average out, so their brightness appears steady — no twinkling.
(c) No. In outer space, there is no atmosphere. Without atmospheric layers to cause refraction, starlight would travel in a straight line without fluctuation. Stars would appear as steady points of light and would not twinkle.Answer: (a) Stars Twinkling: Stars bohot दूर हैं और point sources ki तरह दिखते हैं। उनका light temperature और density बदलते atmosphere के layers से गुज़रता है, jisse continuous refraction होता है। Star ki apparent position fluctuate होती है और आँख तक आने wali light ki quantity बदलती है — jisse twinkling होती है।
(b) Planets: Planets Earth के kafi पास हैं और extended sources (बड़े source) ki तरह दिखते हैं। वह multiple point sources के sum ki तरह हैं, isliye individual fluctuations cancel/average out हो जाती हैं और planets twinkle nahi करते।
(c) Space view: Nahi। Space mein कोई atmosphere nahi होता। बिना atmospheric layers के refraction nahi hoga, isliye starlight straight travel karegi और stars steady points ki तरह बिना twinkle किए dikhenge।
Q8. Explain the phenomenon of dispersion of white light through a glass prism. Draw a neat labeled diagram. Why is the sequence of colours in a rainbow VIBGYOR? [CBSE 2021]Q8. Glass prism से white light के dispersion के process को explain करें। Diagram बनाएं। Rainbow mein colors का sequence VIBGYOR क्यों होता है? [CBSE 2021]
Answer: When white light enters a glass prism, it splits into a band of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). This is called dispersion.
Why VIBGYOR order: Different colours have different wavelengths. Violet has the shortest wavelength and bends the most. Red has the longest wavelength and bends the least. Therefore, violet appears at the bottom of the dispersed spectrum (nearest to base) and red at the top. The sequence VIBGYOR is from the most deviated to the least deviated colour.
In a rainbow, the same order (VIBGYOR) appears because water droplets act as tiny prisms that disperse sunlight through refraction, internal reflection, and further refraction.Answer: जब white light glass prism mein enter करती है, तो ये 7 colors (VIBGYOR) के spectrum mein split हो जाती है। Ise dispersion (वर्ण-विक्षेपण) kehte hain।
VIBGYOR Order ka reason: Different colors ki different wavelengths होती हैं। Violet ki wavelength sabse short होती है और ये sabse ज़्यादा bend होता है। Red ki wavelength sabse long होती है और ये sabse कम bend होता है। Isliye violet spectrum के sabse bottom पर (base के पास) और red sabse top पर दिखता है। VIBGYOR sequence most deviated से least deviated color का order है।
Rainbow mein भी yahi same order दिखता है kyunki water droplets tiny prisms ki तरह sunlight का refraction, internal reflection और dispersion करती हैं।
5 Marks Questions (Long Answer)
Q9. (a) What is myopia? State two causes of myopia. Draw ray diagrams to show (i) the myopic eye and (ii) correction of myopia. (b) The far point of a myopic person is 80 cm. Calculate the power of the lens required. [CBSE 2023]Q9. (a) Myopia kya है? Iske do causes बताएं। Myopic eye और uske correction का ray diagram बनाएं। (b) Ek myopic person का far point 80 cm है। Required lens ki power calculate करें। [CBSE 2023]
Answer: (a)Myopia (near-sightedness) is a defect of vision in which a person can see nearby objects clearly but cannot see distant objects distinctly.
Two causes: (i) Excessive curvature of the eye lens (too converging). (ii) Elongation (lengthening) of the eyeball.
In a myopic eye, the image of distant objects forms in front of the retina instead of on it. This is corrected by placing a concave lens of suitable focal length in front of the eye. The concave lens first diverges the light rays, so that after refraction by the eye lens, the image forms exactly on the retina.
(b) Calculation: Far point = 80 cm = 0.8 m The concave lens must create a virtual image of a distant object at the far point. $u = -\infty$ (object at infinity), $v = -80$ cm = $-0.8$ m $1/f = 1/v - 1/u = 1/(-0.8) - 0 = -1.25$ $f = -0.8$ m Power $P = 1/f = $ $-1.25$ DAnswer: (a) Myopia: Myopia (निकट-vision defect) vision ka wo defect hai jisme person paas ki objects toh clear dekh sakta hai par door ki objects clear nahi dekh pata।
Do causes (कारण): (i) Eye lens ki curvature का excessive (bahut ज़्यादा) hona। (ii) Eyeball का elongated (लंबा) हो jana।
Myopic eye mein distant object ki image retina के आगे (front) बनती है। इसको correct करने के liye spectacles mein ek concave lens (अवतल लेंस) use kiya jata hai। Concave lens rays ko pehle hi diverge kar deta hai taaki eye lens se pass hone ke baad focus retina par bane।
(b) Calculation: Far point = 80 cm = 0.8 m Concave lens infinity पर rakhi object ki virtual image far point (80 cm) पर बनाएगा। $u = -\infty$, $v = -80$ cm = $-0.8$ m $1/f = 1/v - 1/u = 1/(-0.8) - 0 = -1.25$ $f = -0.8$ m Power $P = 1/f = $ $-1.25$ D
Q10. (a) What is Rayleigh's law of scattering? (b) Using this law, explain: (i) why the sky is blue, (ii) why the Sun appears reddish at sunrise and sunset, and (iii) why danger signals are red. [CBSE 2020]Q10. (a) Rayleigh's law of scattering kya है? (b) Is law के base पर explain करें: (i) sky blue क्यों है, (ii) sunrise/sunset पर Sun reddish क्यों दिखता है, और (iii) danger signals red क्यों होते हैं। [CBSE 2020]
Answer: (a) Rayleigh's Law: The amount of scattering of light is inversely proportional to the fourth power of the wavelength of light: Scattering ∝ 1/λ4. Shorter wavelengths scatter much more intensely than longer wavelengths.
(b)(i) Blue sky: Fine atmospheric molecules (N2, O2) scatter shorter wavelengths (blue, violet) much more than longer wavelengths (red, orange). The scattered blue light reaches our eyes from all directions across the sky, making it appear blue. (Violet scatters even more but our eyes are less sensitive to it, and upper atmosphere absorbs some.)
(b)(ii) Reddish sunrise/sunset: Near the horizon, sunlight must travel through a much thicker layer of atmosphere. During this long path, most blue and shorter wavelengths are scattered away by atmospheric particles. Only longer wavelengths (red, orange) survive the journey and reach our eyes, making the Sun appear reddish.
(b)(iii) Red danger signals: Red light has the longest wavelength in the visible spectrum. According to Rayleigh's law, it scatters the least. Therefore, red light can travel the farthest distance through fog, dust, and mist without being scattered. This makes red signals visible from the greatest distance — ideal for danger and traffic signals.Answer: (a) Rayleigh's Law: Light ki scattering का amount uski wavelength ki fourth power के inversely proportional होता है: Scattering $\propto 1/\lambda^4$। Short wavelengths (blue/violet) long wavelengths (red/orange) के comparison mein kafi ज़्यादा scatter होती हैं।
(b)(i) Blue sky: Atmosphere के tiny molecules ($N_2$, $O_2$) short wavelength (blue/violet) light को long wavelength (red/orange) के relative kafi effective scatter करते हैं। Scattered blue light सभी directions से हमारी आँखों तक पहुंचती है, jisse sky blue दिखता है। (Violet ज़्यादा scatter होता है पर eye sensitivity blue के liye ज़्यादा होती है)।
(b)(ii) Reddish sunrise/sunset: Horizon पर sunlight को atmosphere ki मोटी layer से लंबी distance travel karni padti है। Is लंबे path पर blue और short wavelengths scatter हो जाती हैं। सिर्फ long wavelength (red/orange) ही हम तक पहुंच paati है, jisse Sun reddish दिखता है।
(b)(iii) Red danger signals: Red light ki wavelength visible range mein longest होती है, isliye Rayleigh's law के according ये sabse कम scatter होती है। ये fog, dust और mist के threw बिना scatter hue sabse दूर तक jaa सकती है, jisse danger signals दूर से भी clearly visible होते हैं।
☑ What You Can Now Do:☑ What You Can Now Do (अब आप क्या कर सकते हैं):
Label the parts of the human eye and explain how each part helps in vision.Human eye के parts को label कर सकते हैं और उनके function को explain कर सकते हैं।
Explain the power of accommodation and why we can't focus objects closer than 25 cm.Power of accommodation को explain कर सकते हैं और ये बता सकते हैं ki 25 cm से पास focus क्यों nahi होता।
Identify myopia, hypermetropia, and presbyopia — their causes, effects, and corrective lenses.Myopia, hypermetropia और presbyopia के causes, effects और corrective lenses को identify कर सकते हैं।
Calculate the power of a corrective lens given the far point or near point.Far point या near point दिए hone पर corrective lens ki power calculate कर सकते हैं।
Trace a light ray through a prism and identify angle of incidence, emergence, and deviation.Prism से refraction trace कर सकते हैं और angle of incidence, emergence, deviation को label कर सकते हैं।
Explain dispersion, name all 7 VIBGYOR colours, and describe Newton's recombination experiment.Dispersion को explain कर सकते हैं, सातों रंगों के नाम और Newton का recombination experiment बता सकते हैं।
Explain why stars twinkle, planets don't, and why the sun is visible before actual sunrise.Stars के twinkling, planets के steady light और advance sunrise के reasons explain कर सकते हैं।
Use Rayleigh's law (scattering ∝ 1/λ4) to explain blue sky, red sunset, white clouds, and red danger signals.Rayleigh's law ($1/\lambda^4$) के threw blue sky, reddish sunset, white clouds, और red danger signals को explain कर सकते हैं।
