BharatNotes Why this chapter matters for UPSC: This is Chapter 10, "The Human Eye and the Colourful World", of NCERT's Class X Science (Reprint 2026-27). It joins two halves. The first treats the eye as an optical instrument: how it focuses, why it fails (myopia, hypermetropia, presbyopia, cataract) and which lens puts it right. The second explains the optics of the sky: the prism and the rainbow, twinkling stars and an early sunrise, the blue sky and the red danger light. Prelims asks the phenomenon-and-cause pairs directly; GS2 and GS3 meet the eye again as a public-health question about avoidable blindness.
Contemporary hook: WHO's fact sheet of 10 February 2026 says "at least 2.2 billion people have a near or distance vision impairment", and that in at least 1 billion of these cases the impairment could have been prevented or has yet to be addressed. It names the leading causes as refractive errors and cataracts: the two conditions this chapter explains, and the two a pair of spectacles or a short operation can fix.
🧠 First Principles — Read This First
- The eye is a camera. The cornea and the lens form a real, inverted image on the retina; light-sensitive cells turn it into electrical signals that the optic nerves carry to the brain.
- The cornea does most of the bending. NCERT says most of the refraction happens at the outer surface of the cornea; the eye lens only fine-tunes the focal length (p. 161).
- Accommodation has limits. The ciliary muscles change the lens's focal length, but it cannot be made shorter than a certain minimum. That is why a normal eye sees clearly only from about 25 cm out to infinity.
- A defect is a focusing error. In myopia the image of a distant object falls in front of the retina; in hypermetropia the image of a near object falls behind it. A concave or a convex lens of the right power moves the image back onto the retina.
- White light is a mixture. A prism separates the colours because each bends by a different amount: red the least, violet the most.
- The atmosphere does two different things to light. It refracts it (twinkling stars, an early sunrise, a flattened Sun) and it scatters it (the Tyndall effect, the blue sky, red danger signals). Most exam traps mix the two up.
PART 1 — Quick Reference
Table 1: Parts of the human eye (10.1)
| Part | What NCERT says it does |
|---|---|
| Cornea | Thin membrane through which light enters; the transparent bulge on the front of the eyeball. Most of the refraction occurs at its outer surface |
| Eyeball | Approximately spherical, with a diameter of about 2.3 cm |
| Iris | A dark muscular diaphragm behind the cornea that controls the size of the pupil |
| Pupil | Regulates and controls the amount of light entering the eye |
| Eye lens (crystalline lens) | Fibrous, jelly-like material; "merely provides the finer adjustment of focal length" and forms an inverted real image on the retina |
| Ciliary muscles | Change the curvature, and so the focal length, of the eye lens |
| Retina | A delicate membrane with an enormous number of light-sensitive cells, which generate electrical signals when illuminated |
| Optic nerves | Carry the signals to the brain, which interprets them so that we perceive objects as they are |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, sections 10.1-10.1.1 (pp. 161-162).
Table 2: Accommodation, near point and far point (10.1.1)
| Term | What NCERT says |
|---|---|
| Looking at a distant object | Ciliary muscles relaxed, lens thin, focal length increases |
| Looking at a nearby object | Ciliary muscles contract, curvature increases, lens thicker, focal length decreases |
| Accommodation | "The ability of the eye lens to adjust its focal length is called accommodation." The focal length cannot be decreased below a certain minimum limit |
| Near point (least distance of distinct vision) | The minimum distance at which objects are seen most distinctly without strain: about 25 cm for a young adult with normal vision |
| Far point | The farthest point up to which the eye sees objects clearly: infinity for a normal eye |
| Range of a normal eye | Between 25 cm and infinity |
| Cataract | The crystalline lens of people at old age sometimes becomes "milky and cloudy", causing partial or complete loss of vision; cataract surgery can restore vision |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, section 10.1.1 (p. 162).
Table 3: Defects of vision and their correction (10.2)
| Defect | What the person cannot see clearly | Where the image falls | Causes NCERT gives | Correction |
|---|---|---|---|---|
| Myopia (near-sightedness) | Distant objects; the far point is nearer than infinity, perhaps a few metres | In front of the retina | Excessive curvature of the eye lens; elongation of the eyeball | Concave lens of suitable power |
| Hypermetropia (far-sightedness) | Nearby objects; the near point is farther than 25 cm | Behind the retina | Focal length of the eye lens too long; eyeball too small | Convex lens of appropriate power |
| Presbyopia | Nearby objects, as the near point gradually recedes with age | Not specified | Gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens | Corrective eye-glasses |
| Myopia and hypermetropia together | Both near and distant objects | Not specified | Not specified | Bi-focal lens: the upper part concave (distant vision), the lower part convex (near vision) |
NCERT adds that refractive defects can now also be corrected with contact lenses or through surgical interventions (p. 164).
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, section 10.2 and Figs. 10.2-10.3 (pp. 162-164).
Table 4: NCERT's correction numericals (Exercises 5-7)
Both formulae come from Chapter 9: the lens formula 1/v − 1/u = 1/f, and power P = 1/f with f in metres.
| Exercise | Data | Working | NCERT's answer |
|---|---|---|---|
| 5 | −5.5 D lens for distant vision; +1.5 D for near vision | f = 1/P: 1/(−5.5) = −0.18 m; 1/(+1.5) = +0.67 m | (i) −0.18 m; (ii) +0.67 m |
| 6 | Far point of a myopic eye 80 cm | The lens must make an object at infinity appear at the far point: u = −∞, v = −80 cm, so f = −80 cm = −0.8 m and P = −1.25 D | Concave lens; −1.25 D |
| 7 | Near point of a hypermetropic eye 1 m; normal near point 25 cm | An object at u = −25 cm must appear at v = −100 cm: 1/f = −1/100 + 1/25 = 3/100, so f = +33.3 cm and P = +3.0 D | Convex lens; +3.0 D |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, Exercises 5-7 (p. 170); NCERT Answers (p. 219). The working is arithmetic on Chapter 9's formulae; the Answers print only the results.
Table 5: Prism, dispersion and the rainbow (10.3-10.4)
| Term | What NCERT says |
|---|---|
| Glass slab | Parallel refracting surfaces: the emergent ray is parallel to the incident ray but "slightly displaced laterally" |
| Triangular prism | Two triangular bases and three rectangular lateral surfaces, inclined to each other |
| Angle of the prism (∠A) | The angle between its two lateral faces |
| Rays in Fig. 10.4 | PE incident ray, EF refracted ray, FS emergent ray; ∠i incidence, ∠r refraction, ∠e emergence |
| Bending at each face | Air to glass at face AB: towards the normal. Glass to air at face AC: away from the normal |
| Angle of deviation (∠D) | The angle between the emergent ray and the direction of the incident ray, caused by the prism's shape |
| Spectrum | The band of coloured components of a light beam: violet, indigo, blue, green, yellow, orange, red (VIBGYOR) |
| Dispersion | The splitting of light into its component colours. "The red light bends the least while the violet the most" |
| Newton | The first to use a glass prism to obtain the spectrum of sunlight. A second identical prism, inverted, recombined the colours into white light |
| White light | "Any light that gives a spectrum similar to that of sunlight" |
| Rainbow | Dispersion of sunlight by tiny water droplets acting like small prisms: they refract and disperse sunlight, reflect it internally, and refract it again on the way out. Always formed opposite the Sun; also seen through a waterfall or fountain with the Sun behind you |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, sections 10.3-10.4, Activities 10.1-10.2, Figs. 10.4-10.8 (pp. 165-167).
Table 6: Atmospheric refraction (10.5)
| What you see | NCERT's explanation |
|---|---|
| Objects waver above a fire or radiator | The hotter air is lighter (less dense) and has a refractive index slightly less than the cooler air above; the conditions keep changing, so the apparent position keeps shifting |
| Stars twinkle | Starlight is refracted continuously in a medium of gradually changing refractive index. Stars are so distant that they act as point-sized sources, so as the path varies, the light entering the eye flickers |
| A star near the horizon looks higher | The atmosphere bends starlight towards the normal |
| Planets do not twinkle | They are closer and are seen as extended sources; the variations from their many point-sized sources average out to zero |
| Advance sunrise and delayed sunset | The Sun is visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset |
| Flattened Sun at sunrise and sunset | The same atmospheric refraction |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, section 10.5 and Figs. 10.9-10.10 (p. 168).
Table 7: Scattering of light (10.6)
| What you see | NCERT's explanation |
|---|---|
| Path of a beam visible in a colloidal solution but not in a true solution | The particles of a colloid are relatively larger and scatter light |
| Beam of sunlight in a smoke-filled room; sunlight through a dense forest canopy | Tyndall effect: scattering by fine particles (smoke, tiny water droplets, dust, air molecules); in the forest, tiny water droplets in the mist scatter light |
| Colour of scattered light | Depends on particle size: very fine particles scatter mainly blue; larger particles scatter longer wavelengths; large enough particles make the scattered light appear white |
| Clear sky is blue | Air molecules and fine particles are smaller than the wavelength of visible light and scatter blue more strongly than red. Red light's wavelength is about 1.8 times that of blue |
| Sky dark without an atmosphere; dark at very high altitudes | No scattering, or too little. The same reasoning answers Exercise 12 on the astronaut |
| Danger signal lights are red | "The red is least scattered by fog or smoke", so it is seen in the same colour at a distance |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, section 10.6 (p. 169) and Exercise 12 (p. 170).
Table 8: Eye donation (NCERT's box)
| Point | What NCERT says |
|---|---|
| Need | About 35 million people in the developing world are blind, most of them curable; about 4.5 million people with corneal blindness can be cured by corneal transplantation of donated eyes, and 60% of them are children below the age of 12 |
| Who can donate | Any age group or sex; people who use spectacles or have been operated for cataract; people with diabetes, hypertension or asthma, and those without communicable diseases |
| Who cannot | Persons infected with, or who died of, AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis |
| Timing | Eyes must be removed within 4-6 hours after death; inform the nearest eye bank immediately |
| Procedure | The eye bank team removes the eyes at home or at a hospital; it takes 10-15 minutes and causes no disfigurement |
| Eye bank | Collects, evaluates (against strict medical standards) and distributes donated eyes; eyes unsuitable for transplant go to research and medical education; donor and recipient identities stay confidential |
| Yield | "One pair of eyes gives vision to up to FOUR CORNEAL BLIND PEOPLE." |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27, box on eye donation (pp. 164-165).
Table 9: NCERT lines to read with care
| Where | What the book prints | How to read it |
|---|---|---|
| Section 10.2, opening (p. 162) | The defects follow when the eye "may gradually lose its power of accommodation" | NCERT's own causes for myopia (lens curvature, a longer eyeball) and hypermetropia (focal length too long, a small eyeball) are not a loss of accommodation; only presbyopia is tied to the power of accommodation decreasing with age (p. 163) |
| Section 10.6, introduction (p. 169) | Lists "the reddening of the sun at sunrise and the sunset" among scattering phenomena | The current chapter never explains it. The explanation was the 2020-21 edition's section 11.6.3, kept below as Retained content |
| Section 10.3 (p. 165) | The glass-slab ray is "slightly displaced laterally" | Chapter 9 says the same ray is shifted sideward; both describe one effect, the lateral displacement |
| What you have learnt (p. 170) | Myopia as short-sightedness | The body text calls it near-sightedness: the same defect |
| Eye-donation box (pp. 164-165) | 35 million blind; 4.5 million with corneal blindness | The box gives no year or source for these figures. Quote them as NCERT's, not as current data |
Source: NCERT, Science Class X, ch. 10, Reprint 2026-27 (pp. 162-170); ch. 9 (p. 147).
PART 2 — Concepts & Narrative
The eye as a camera (10.1)
NCERT calls the eye the most significant sense organ: with the eyes shut we can still identify objects by smell, taste, sound or touch, but not their colours. Its optics are those of Chapter 9. The cornea and the lens form a real, inverted image on the retina, and the brain interprets the signals that the optic nerves carry from it (Table 1).
Cornea or lens: which focuses? Both refract, but not equally. NCERT says "Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea", while the crystalline lens provides only the finer adjustment needed to focus objects at different distances. The cornea does the heavy, fixed bending; the lens does the small, variable part.
Accommodation and its limit (10.1.1)
Accommodation has a floor: the focal length of the lens cannot be decreased below a certain minimum, and that floor sets the near point of about 25 cm. Two NCERT exercises rest on it. Exercise 8 asks why a normal eye cannot see clearly an object closer than 25 cm: the lens cannot shorten its focal length enough to bring that image onto the retina. Exercise 9 asks what happens to the image distance in the eye as an object moves farther away. The image must still fall on the retina, so the image distance stays the same; it is the focal length that changes, growing as the ciliary muscles relax.
Defects of vision and their correction (10.2)
Each defect in Table 3 is a focusing error with a physical cause. A lens that is too curved, or an eyeball that is too long, puts the image of a distant object in front of the retina (myopia). A focal length that is too long, or an eyeball that is too small, puts the image of a near object behind it (hypermetropia). Weakening ciliary muscles and a less flexible lens push the near point outwards with age (presbyopia). The corrective lens moves the image back onto the retina.
How to choose the corrective lens in a numerical. The lens's job is to make the object you want to see appear at a point the faulty eye can already see. For myopia, an object at infinity must appear at the far point, so v = −(far point) and u = −∞; the focal length equals minus the far point, and the lens is concave (Exercise 6: far point 80 cm gives −1.25 D). For hypermetropia, an object at 25 cm must appear at the faulty near point, so u = −25 cm and v = −(near point); the lens comes out convex (Exercise 7: near point 1 m gives +3.0 D). A negative power always means a concave, diverging lens.
Refraction through a prism (10.3)
A glass slab's parallel faces send the ray out parallel to its original path. A prism's two refracting faces instead meet at the angle of the prism. In Activity 10.1 the ray bends towards the normal on entering at face AB and away from it on leaving at face AC, and because the faces are not parallel, the emergent ray leaves at an angle to the incident ray: the angle of deviation.
Dispersion, Newton and the rainbow (10.4)
Dispersion happens because each colour bends through a different angle, red the least and violet the most, so each leaves the prism along its own path (Activity 10.2). Newton's second prism split no colour further, while an identical prism placed upside down recombined the spectrum into white light; this gave him the idea that sunlight is made up of seven colours. A raindrop acts as a small prism that also reflects: it refracts and disperses sunlight, reflects it internally and refracts it again on the way out, which is why a rainbow appears only on the side of the sky opposite the Sun.
Atmospheric refraction (10.5)
The air is not a uniform medium. Hotter, lighter air has a slightly lower refractive index than the cooler air around it, and moving air keeps changing it. Objects seen above a fire therefore waver, and starlight, bent towards the normal as it passes through layers of gradually changing refractive index, keeps shifting in apparent position and brightness.
Why stars twinkle and planets do not. A star is so distant that it is effectively a point source of light, so every wobble of its light path shows. A planet is much closer and appears as an extended source, a disc made of many point sources; their individual fluctuations average out to zero. The same refraction gives an early sunrise and a late sunset: the Sun is visible about 2 minutes before it actually crosses the horizon and about 2 minutes after it actually sets, and its disc looks flattened near the horizon.
Scattering: the Tyndall effect, the blue sky and the red signal (10.6)
One rule covers the section: the colour of scattered light depends on the size of the particles. Air molecules are smaller than the wavelength of visible light and scatter blue more strongly than red, so scattered blue reaches the eye from every direction and the sky looks blue. Larger particles scatter longer wavelengths, and large enough ones make the scattered light look white. Without an atmosphere there is nothing to scatter, and the sky looks dark, as it does at very high altitudes and to an astronaut. Red, the least scattered by fog or smoke, keeps its colour at a distance, which is why danger signals are red. The Tyndall effect, the visible path of a beam through smoke or forest mist, is the same scattering that Class IX used to tell a colloid from a true solution.
Refraction or scattering? A quick sort. Twinkling of stars, a star appearing higher near the horizon, the early sunrise and late sunset, the flattened Sun and the shimmer over a fire are all refraction by air of changing density. The Tyndall effect, the blue sky, the dark sky in space, the white look of large-particle scattering and the visibility of red signals are all scattering. The rainbow is neither alone: it is dispersion, refraction and internal reflection inside raindrops.
Did the 2020-21 edition differ?
Yes. The chapter was Chapter 11 (pp. 187-198) in the 2020-21 edition and is Chapter 10 (pp. 161-170) now. Four pieces have gone: a "Do You Know?" box on the visual system and the pupil (p. 188); a box on why we have two eyes (p. 189); section 11.6.3 on the colour of the Sun at sunrise and sunset, with Activity 11.3 (pp. 196-197); and the exercise asking why the Sun appears reddish early in the morning. The summary line on scattering, which used to end with the reddening of the Sun at sunrise and sunset, now names only the blue colour of the sky. The rest is kept, including the eye-donation box, bi-focal lenses, atmospheric refraction and the four MCQs.
Source: NCERT, Science Class X, 2020-21 edition (whole-book zip, Wayback Machine capture of 9 October 2021), compared with Reprint 2026-27.
Retained content: Colour of the Sun at sunrise and sunset, and two boxes on the eye (2020-21 edition, ch. 11, pp. 188-197; not in Reprint 2026-27)
The pupil as an aperture (p. 188). The pupil "acts like a variable aperture whose size can be varied with the help of the iris". In very bright light the iris contracts the pupil to let in less light; in dim light it expands the pupil to let in more. That is why, stepping from bright light into a dim room, you cannot see clearly for a while.
Why two eyes (p. 189). A human has "a horizontal field of view of about 150° with one eye and of about 180° with two eyes", and two detectors see faint objects better than one. Many prey animals have eyes on opposite sides of the head for the widest view; ours face forward, trading some field of view for stereopsis. Each eye sees a slightly different image, and the brain combines them to judge depth: "Shut one eye and the world looks flat".
Colour of the Sun at sunrise and sunset (11.6.3, pp. 196-197). In Activity 11.3, a parallel beam of white light passes through a glass tank of water. Dissolving about 200 g of sodium thiosulphate (hypo) in about 2 L of water and adding 1 to 2 mL of concentrated sulphuric acid precipitates fine sulphur particles in about 2 to 3 minutes. Blue light is then seen from three sides of the tank, scattered by the minute colloidal sulphur particles, while the transmitted beam on the screen turns "at first the orange red colour and then bright crimson red colour". The sky works the same way. Light from the Sun near the horizon passes through "thicker layers of air and larger distance" in the atmosphere, so most of the blue and shorter wavelengths are scattered away and the light that reaches us is reddish. At noon the path is shorter, and "the Sun appears white as only a little of the blue and violet colours are scattered".
Source: NCERT, Science Class X, 2020-21 edition, ch. 11, "Do You Know?" box (p. 188), box on two eyes (p. 189), section 11.6.3 and Activity 11.3 (pp. 196-197).
Beyond the textbook: what the 2021-22 Class XII Physics book added
- Rayleigh scattering. "The amount of scattering is inversely proportional to the fourth power of the wavelength. This is known as Rayleigh scattering." Combined with Class X's statement that red light's wavelength is about 1.8 times blue's, this means blue is scattered roughly 1.8⁴ ≈ 10.5 times as strongly as red (arithmetic on the two books, printed in neither).
- Why blue and not violet. Violet, with a still shorter wavelength, is scattered even more than blue, "But since our eyes are more sensitive to blue than violet, we see the sky blue."
- White clouds. Rayleigh's law holds for particles much smaller than the wavelength. Large scatterers such as raindrops and large dust or ice particles scatter all wavelengths nearly equally, so clouds are generally white.
- Rainbow angles. In the primary rainbow, violet light emerges at about 40° and red at about 42° to the incoming sunlight after one internal reflection, so red is at the top. A secondary rainbow comes from two internal reflections; it is fainter and its colours are reversed. An observer sees a rainbow only with their back to the Sun.
- Size of atmospheric refraction. The refractive index of air with respect to vacuum is 1.00029. It shifts the Sun's apparent direction by about half a degree, and since the Earth turns 1° in 4 minutes, sunrise comes about 2 minutes early and sunset about 2 minutes late. The same book adds the reddish sun and full moon near the horizon.
Source: NCERT, Physics Part II, Textbook for Class XII, ch. 9 "Ray Optics and Optical Instruments", 2021-22 edition, section 9.3 and Example 9.5 (p. 318), sections 9.7.1-9.7.2 (pp. 333-335). These passages are not in the current Class XII book.
Beyond the textbook: vision loss in India and the world
- India's national survey. The National Blindness and Visual Impairment Survey 2015-2019 was planned by the Ministry of Health and Family Welfare; Dr Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS, New Delhi, ran the field work, analysis and report. People aged 50 and above were surveyed house to house with the Rapid Assessment of Avoidable Blindness (RAAB) method in 31 districts of 24 States and Union Territories, from September 2015 to June 2018, with a sample of 3,000 per district (93,000 in all). A further survey of the 0-49 age group covered six districts in January-February 2019.
- What it found. Prevalence of blindness was 0.36% across all ages ("Nearly one in 300 persons are blind in India"), 0.052% among those aged 0-49 and 1.99% among those aged 50 and above.
- The world (WHO, 10 February 2026). At least 2.2 billion people have a near or distance vision impairment, and in at least 1 billion of these it could have been prevented or has not been addressed. Within that 1 billion, distance impairment or blindness is caused mainly by cataract (94 million), refractive error (88.4 million), age-related macular degeneration (8 million), glaucoma (7.7 million) and diabetic retinopathy (3.9 million); near-vision impairment mainly by presbyopia (826 million). "1 in 2 people globally who need cataract surgery don't have access to that surgery."
- Myopia is spreading. A 2016 meta-analysis of 145 studies (2.1 million participants) estimated 1406 million people with myopia in 2000 (22.9% of the world population) and projected 4758 million (49.8%) by 2050, with 938 million (9.8%) highly myopic. It defined myopia as −0.50 dioptre or less and high myopia as −5.00 D or less. These are projections, not counts.
Source: India Vision Atlas (NPCB&VI and AIIMS), survey page and home page, read via the Wayback Machine; WHO fact sheet "Blindness and vision impairment", 10 February 2026; Holden BA et al., Ophthalmology 2016;123(5):1036-42.
PART 3 — UPSC Integration
Cross-paper relevance
- Prelims (general science) — Phenomenon and cause: twinkling and the early sunrise (atmospheric refraction), blue sky and red signals (scattering), rainbow (dispersion with internal reflection), Tyndall effect. The lens for each defect, the 25 cm near point, VIBGYOR and which colour bends most.
- GS2 (health) — Avoidable blindness: cataract and uncorrected refractive error as leading causes, access to cataract surgery and spectacles, eye donation and eye banks, survey evidence for policy.
- GS3 (science and technology) — The eye as an optical instrument; lens power in dioptres; atmospheric optics as applied physics.
Past questions on these themes: No past question in the bank is set directly on this chapter. The closest is GS3 2017 on stem cell therapy, which lists a damaged cornea among the conditions it treats: an adjacent theme.
Frames for Mains Answers
1. Avoidable blindness as a public-health problem. Begin with the physics: a cloudy lens (cataract) and a focusing error (refractive error) both block the image from the retina, and both are correctable by surgery or a lens. Then the scale: WHO puts these as the leading causes of vision impairment, and India's 2015-19 survey found blindness concentrated among people aged 50 and above (1.99% against 0.36% overall). Close on access: WHO estimates that half of those who need cataract surgery cannot get it, and corneal blindness depends on eye donation.
2. Why fog, smoke and the sky look the way they do. Use NCERT's particle-size rule: fine particles scatter blue, larger ones longer wavelengths, large ones scatter all colours and look white. Apply it to the blue sky, white clouds, the choice of red for danger signals, and visibility in mist or smoke.
3. Myopia as an emerging concern. Define it (image in front of the retina, corrected by a concave lens), then cite the 2016 projection of nearly half the world being myopic by 2050, stated as a projection. Link it to screening and spectacle access, since uncorrected refractive error remains a leading cause of impairment in every country (WHO).
Exam Strategy
Prelims fact-traps:
- Most refraction in the eye happens at the cornea, not the lens.
- Relaxed ciliary muscles mean a thin lens, a longer focal length and distant vision.
- Myopia: image in front of the retina, concave lens. Hypermetropia: image behind the retina, convex lens.
- Presbyopia comes from weak ciliary muscles and a less flexible lens with age, not from eyeball length.
- Red bends least in a prism and is least scattered; violet bends most.
- Twinkling is refraction, not scattering; planets do not twinkle because they are extended sources.
- A rainbow is always on the side of the sky opposite the Sun.
Mains: Use the chapter for the science line of a GS2 health answer on avoidable blindness: one mechanism from NCERT, one number from the national survey or WHO with its date, then the access gap.
Practice Questions
Questions 1-4 are the NCERT exercise MCQs. Practice (UPSC-pattern, not past papers): questions 5-10.
1. The human eye can focus on 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-sightedness
Answer: (b) (NCERT Answers, p. 219).
2. The human eye forms the image of an object at its
(a) cornea
(b) iris
(c) pupil
(d) retina
Answer: (d) (NCERT Answers, p. 219).
3. 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 m
Answer: (c) (NCERT Answers, p. 219).
4. The change in focal length of an eye lens is caused by the action of the
(a) pupil
(b) retina
(c) ciliary muscles
(d) iris
Answer: (c) (NCERT Answers, p. 219). For Exercises 5-7 the key reads: 5 (i) −0.18 m, (ii) +0.67 m; 6 concave lens, −1.25 D; 7 convex lens, +3.0 D.
5. Consider the following statements:
1. The twinkling of stars is caused by the scattering of starlight by air molecules.
2. Planets do not twinkle because they are seen as extended sources of light.
3. The Sun is visible for a short time before it actually crosses the horizon at sunrise.
Which of the statements given above is/are correct?
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Answer: (c). Twinkling is due to atmospheric refraction, not scattering (p. 168).
6. Consider the following pairs (defect : a cause NCERT gives : correction):
1. Myopia : elongation of the eyeball : concave lens
2. Hypermetropia : eyeball too small : concave lens
3. Presbyopia : weakening of the ciliary muscles : corrective eye-glasses
Which of the pairs given above is/are correctly matched?
(a) 1 only
(b) 2 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (c). Hypermetropia is corrected with a convex lens (pp. 163-164).
7. The far point of a myopic eye is 2 m. The lens needed to correct it has a power of
(a) +0.5 D
(b) −0.5 D
(c) +2.0 D
(d) −2.0 D
Answer: (b). The lens must image infinity at the far point, so f = −2 m and P = 1/f = −0.5 D, a concave lens (arithmetic, on the method of Exercise 6).
8. Red is used for danger signals mainly because red light
(a) has the shortest wavelength of visible light
(b) is bent the most by a prism
(c) is the least scattered by fog or smoke
(d) is absorbed least by the eye lens
Answer: (c). Red has the longest wavelength and bends least in a prism (pp. 167, 169).
9. Which one of the following is explained by scattering of light rather than by refraction?
(a) Twinkling of stars
(b) Flattening of the Sun's disc at sunset
(c) The sky appearing dark to an astronaut
(d) The wavering of objects seen above a fire
Answer: (c). Without an atmosphere there is nothing to scatter sunlight; the other three are atmospheric refraction (pp. 168-169).
10. With reference to the National Blindness and Visual Impairment Survey 2015-2019, consider the following statements:
1. It was planned by the Ministry of Health and Family Welfare, with field work by AIIMS, New Delhi.
2. It found the prevalence of blindness across all ages to be 0.36%.
3. It covered only people aged 50 years and above.
Which of the statements given above is/are correct?
(a) 1 and 2 only
(b) 2 and 3 only
(c) 1 and 3 only
(d) 1, 2 and 3
Answer: (a). The main RAAB survey covered people aged 50 and above, but a separate survey of the 0-49 age group was held in January-February 2019 (India Vision Atlas).
📦 Revision Capsule
Hard Facts
- Eyeball diameter about 2.3 cm; most refraction at the cornea's outer surface.
- Near point about 25 cm (young adult); far point infinity; normal range 25 cm to infinity.
- Myopia: concave lens; hypermetropia: convex lens; presbyopia: age, weak ciliary muscles; bi-focal = concave above, convex below.
- Exercises: −5.5 D gives −0.18 m; +1.5 D gives +0.67 m; far point 80 cm needs −1.25 D; near point 1 m needs +3.0 D.
- VIBGYOR; red bends least, violet most; Newton recombined the spectrum with an inverted second prism.
- The Sun is seen about 2 minutes before actual sunrise and after actual sunset.
- Red wavelength about 1.8 times blue (NCERT X); scattering varies as 1/λ⁴ (Class XII, 2021-22).
Core Concepts
- Accommodation changes focal length, never the lens-to-retina distance.
- A corrective lens makes the object appear where the faulty eye can already see.
- Dispersion: each colour bends by a different amount.
- Atmospheric refraction moves and flickers point sources; extended sources average out.
- Particle size decides the colour of scattered light: fine scatters blue, large scatters white.
Confused Pairs
- Myopia (near-sightedness, concave) vs hypermetropia (far-sightedness, convex).
- Presbyopia (age, accommodation) vs hypermetropia (eyeball or focal length).
- Refraction (twinkling, early sunrise) vs scattering (blue sky, red signal).
- Dispersion (prism, rainbow) vs scattering (sky).
- Cornea (most refraction) vs eye lens (fine adjustment).
- Glass slab (parallel shift, no deviation) vs prism (deviation).
Data Points
- NCERT box: 35 million blind in the developing world; 4.5 million with corneal blindness, 60% under 12; eyes removed within 4-6 hours; one pair helps up to four people.
- India, 2015-19 survey: blindness 0.36% overall, 0.052% at 0-49, 1.99% at 50 and above; 31 districts, 24 States/UTs.
- WHO (10 Feb 2026): at least 2.2 billion with vision impairment; cataract 94 million and refractive error 88.4 million among the 1 billion preventable or unaddressed.
- Myopia: 22.9% of the world in 2000; projected 49.8% by 2050.
- Air's refractive index 1.00029; rainbow violet 40°, red 42° (Class XII, 2021-22).
PYQ Pattern
- No past question in the bank matches this chapter directly.
- Adjacent: GS3 2017 on stem cell therapy, which names a damaged cornea among its uses.
Sources
- NCERT, Science, Textbook for Class X, ch. 10 "The Human Eye and the Colourful World", Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, Answers, Reprint 2026-27 — ncert.nic.in PDF.
- NCERT, Science, Class X, 2020-21 edition (whole-book zip), as archived on 9 October 2021, ch. 11 "The Human Eye and the Colourful World" — Wayback Machine.
- NCERT, Physics Part II, Textbook for Class XII, ch. 9 "Ray Optics and Optical Instruments", 2021-22 edition, as archived on 1 October 2021 — Wayback Machine.
- India Vision Atlas (NPCB&VI, AIIMS), "National Blindness and Visual Impairment Survey 2015-2019", Wayback Machine capture of 14 January 2025 — indiavisionatlasnpcb.aiims.edu (archived).
- India Vision Atlas (NPCB&VI, AIIMS), home page (prevalence of blindness 2015-2019), Wayback Machine capture of 14 January 2025 — indiavisionatlasnpcb.aiims.edu (archived).
- World Health Organization, "Blindness and vision impairment", fact sheet, 10 February 2026 — who.int.
- Holden BA, Fricke TR, Wilson DA et al., "Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050", Ophthalmology 2016;123(5):1036-42 — PubMed 26875007.
