The Human Eye and the Colourful World

This is Chapter 10 of NCERT's Class 10 Science textbook (pages 161–170 of the 2026-27 reprint; the contents page and the PDF agree). It has six parts — the human eye and accommodation (10.1), defects of vision (10.2), refraction through a prism (10.3), dispersion and the rainbow (10.4), atmospheric refraction (10.5) and scattering (10.6) — with 2 activities, Figures 10.1–10.10, a ‘Think it over’ box on eye donation, one ‘Questions’ box and 12 exercise questions. It is short, but almost every paragraph is a Prelims favourite. It follows the physics conventions set on the previous chapter's page: every number is computed and asserted in the page's source, and every ray diagram is traced from the formulae or Snell's law.

• Neighbouring chapters: Light – Reflection and Refraction (Chapter 9) supplies the lens formula, power in dioptres and refractive index used throughout. On this site, Sensory, Attentional and Perceptual Processes (Class 11 Psychology) covers what the brain does with the retinal image; Control and Coordination (this book) covers nerves and the brain; Composition and Structure of Atmosphere and Solar Radiation, Heat Balance and Temperature (Class 11 Geography) cover the air that refracts and scatters sunlight.
• Prelims — science: UPSC has asked this chapter's content again and again — the sky looks black to astronauts and from the Moon (1981, 1982, 1987, 1988, 1996), planets don't twinkle (1981, 1996), the retina is the camera's film (1982), the pupil dilates in the dark (1983), a ray diagram of myopia (1986), eye donation uses the cornea (1999, 2001), the rainbow's three processes (1991, 2013), and twinkling and the Sun's size and colour near the horizon (2013).
• GS3 — science and technology: eye health and corrective technologies (spectacles, contact lenses, cataract surgery, LASIK), organ and tissue donation (corneal transplants), and the atmospheric optics behind visibility, haze and signal design.

• Eye = camera: cornea (most refraction) → pupil (size set by the iris) → crystalline lens (fine focus) → real, inverted image on the retina → optic nerve → brain. Eyeball ≈ 2.3 cm across.
• Accommodation: ciliary muscles relax → thin lens, long f (distant objects); contract → thick lens, short f (near objects). Near point ≈ 25 cm, far point = infinity for a normal young eye. Cataract: the lens turns milky; surgery restores vision.
• Myopia (near-sighted): far point closer than infinity, image in front of the retina (lens too curved or eyeball too long) → concave lens. Hypermetropia (far-sighted): near point beyond 25 cm, image behind the retina (f too long or eyeball too short) → convex lens. Presbyopia: age → bifocals (concave above, convex below).
• Prism: light bends towards the normal entering, away leaving, and emerges deviated towards the base by the angle of deviation. Dispersion: white light → VIBGYOR; violet bends most, red least. Newton's inverted second prism recombines them.
• Rainbow: in the direction opposite the Sun; drops refract, disperse, reflect internally and refract again. Atmospheric refraction: stars twinkle (point sources), planets don't (extended); Sun seen ≈ 2 min before sunrise and after sunset; flattened Sun at the horizon.
• Scattering: Tyndall effect (beam visible in smoke, mist in a forest). Fine particles scatter blue more (red's wavelength ≈ 1.8 × blue's) → blue sky; no atmosphere → dark sky (astronauts). Red is scattered least → danger signals.

Of all the senses, NCERT notes, only sight tells us colours. Light enters through the cornea, a thin transparent membrane that forms the bulge at the front of the eyeball; the eyeball is roughly spherical, about 2.3 cm in diameter. Most of the refraction happens at the outer surface of the cornea; the crystalline lens only provides the finer adjustment of focal length for objects at different distances. Behind the cornea, the iris — a dark muscular diaphragm — controls the size of the pupil, which regulates the amount of light entering. The eye lens forms an inverted real image on the retina, a delicate membrane with an enormous number of light-sensitive cells; activated by light, they generate electrical signals that travel by the optic nerves to the brain, which interprets them so we perceive objects as they are.

The eye lens is a fibrous, jelly-like material whose curvature the ciliary muscles can change. When they relax, the lens becomes thin and its focal length increases — we see distant objects clearly. When they contract, the lens's curvature increases, it becomes thicker and its focal length decreases — we see nearby objects. This ability to adjust the focal length is accommodation. The focal length cannot go below a certain minimum: the minimum distance at which objects are seen most distinctly without strain is the least distance of distinct vision or near point — about 25 cm for a young adult with normal vision. The farthest point seen clearly is the far point — infinity for a normal eye. So a normal eye sees clearly from 25 cm to infinity. In old age the crystalline lens can become milky and cloudy — cataract — causing partial or complete loss of vision; cataract surgery restores vision.

When the eye gradually loses its power of accommodation, vision blurs from refractive defects. NCERT lists three. (a) Myopia (near-sightedness): near objects are clear but distant ones are not; the far point is nearer than infinity (perhaps a few metres); the image of a distant object forms in front of the retina. Causes: (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. Correction: a concave lens of suitable power. (b) Hypermetropia (far-sightedness): distant objects are clear but near ones are not; the near point is farther than 25 cm, so reading material must be held well beyond 25 cm; light from a close object focuses behind the retina. Causes: (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small. Correction: a convex lens, which adds the focusing power needed. (c) Presbyopia: with age the near point gradually recedes because the ciliary muscles weaken and the lens loses flexibility. Someone with both near and distance problems often needs bi-focal lenses — the upper part concave (distant vision), the lower part convex (near vision). Refractive defects can also be corrected with contact lenses or surgery.

A triangular glass prism has two triangular bases and three rectangular lateral faces inclined to each other; the angle between two lateral (refracting) faces is the angle of the prism. In Activity 10.1 you trace a prism on paper, fix pins P and Q on a line PE inclined to face AB, sight their images through face AC, fix pins R and S in line with those images, and join up. PE is the incident ray, EF the refracted ray, FS the emergent ray. At AB (air → glass) the ray bends towards the normal; at AC (glass → air) it bends away. Because the faces are not parallel, the emergent ray leaves at an angle to the incident direction — the angle of deviation (D).

In Activity 10.2 sunlight through a narrow slit in cardboard falls on a prism; turning the prism throws a band of colours on a screen: Violet, Indigo, Blue, Green, Yellow, Orange, Red — VIBGYOR. The band is a spectrum; the splitting of light into its component colours is dispersion. It happens because different colours bend through different angles in the prism: red the least, violet the most. Isaac Newton was the first to obtain the spectrum of sunlight with a prism; a second similar prism did not split any colour further, but a second identical prism placed inverted recombined the colours into white light — showing that sunlight is made of these colours. Any light giving a spectrum like sunlight's is called white light.

A rainbow is a natural spectrum seen after a shower, caused by dispersion of sunlight by tiny water droplets. It always forms in the direction opposite to the Sun. The droplets act like small prisms: they refract and disperse sunlight, reflect it internally, and refract it again as it leaves the drop, so different colours reach the observer's eye. You can also see one in the spray of a waterfall or fountain with the Sun behind you.

Air just above a fire or radiator is hotter, lighter and has a slightly lower refractive index than the cooler air above; because the medium keeps changing, objects seen through it appear to waver — atmospheric refraction on a small scale. Twinkling of stars is the same on a large scale: starlight is refracted continuously as it passes through air of gradually changing refractive index, bending towards the normal, so a star near the horizon appears slightly higher than it is. The atmosphere keeps changing, so the apparent position fluctuates; stars are so distant that they are point sources, and the amount of light entering the eye flickers — brighter, fainter: twinkling. Planets are much closer and appear as extended sources; the variations from their many points average out, so they do not twinkle. Advance sunrise and delayed sunset: the Sun is visible about 2 minutes before it actually crosses the horizon and about 2 minutes after it sets; the flattening of the Sun's disc at sunrise and sunset has the same cause.

The atmosphere is a heterogeneous mixture of minute particles — smoke, tiny water droplets, dust and air molecules. When a beam of light strikes such fine particles its path becomes visible: this scattering by colloidal particles is the Tyndall effect (Class 9) — a fine beam of sunlight entering a smoke-filled room through a small hole, or sunlight through the canopy of a dense forest where mist droplets scatter light. The colour scattered depends on particle size: very fine particles scatter mainly blue, larger particles scatter longer wavelengths, and large enough particles scatter all colours, so the light looks white.

Why the clear sky is blue: air molecules and other fine particles are smaller than the wavelength of visible light, so they scatter the shorter wavelengths at the blue end more strongly than the red end; red light's wavelength is about 1.8 times blue's. Scattered blue light reaches our eyes from all over the sky. With no atmosphere there would be no scattering and the sky would look dark — as it does to passengers at very high altitudes. Danger signals are red because red is least scattered by fog or smoke, so it is seen in the same colour from far away.

The chapter has one ‘Questions’ box (page 164) and questions inside its two activities and the text. Outlines follow; every number is computed in the page's source.

Try each question first, then open the outline. Every number below is computed and checked against the book's answer key.

Numbers

One-liners

• Most refraction in the eye happens at the outer surface of the cornea; the lens does the fine adjustment.
• The iris controls the size of the pupil; the pupil controls the light entering.
• The retina receives a real, inverted image; the optic nerve carries signals to the brain.
• Ciliary muscles relaxed → thin lens → distant vision; contracted → thick lens → near vision.
• Cataract: the eye lens becomes milky and cloudy; treated by surgery.
• Myopia → concave lens; hypermetropia → convex lens; presbyopia → bifocals often.
• Bifocals: concave upper part for distance, convex lower part for reading.
• A prism deviates light towards its base; the emergent ray is not parallel to the incident ray.
• Dispersion: red bends least, violet most; Newton recombined colours with an inverted prism.
• Rainbow: opposite the Sun; refraction, dispersion, internal reflection, refraction.
• Stars twinkle because of atmospheric refraction; planets, being extended sources, don't.
• Blue sky: fine particles scatter short wavelengths most; no atmosphere → dark sky.
• Danger signals are red because red is scattered least by fog and smoke.

Traps to avoid

• Myopia is corrected by a CONCAVE (minus) lens, hypermetropia by a CONVEX (plus) lens.
• Muscles relaxed = thin lens = DISTANT vision.
• The image distance in the eye never changes; the focal length does.
• Twinkling and advance sunrise are REFRACTION; blue sky and red sunset are SCATTERING; rainbow is DISPERSION + internal reflection.
• Shorter wavelengths (blue) are scattered MORE.
• Planets don't twinkle; stars seen from a spaceship don't twinkle either.
• Eye donation transplants the CORNEA, not the retina or the lens.
• In a primary rainbow red is on the outside; violet deviates most in a prism.
• Cataract is a cloudy lens, not a focusing defect — spectacles don't cure it.
• Rationalisation gap: the red-sunset explanation is no longer in the book, though the phenomenon is still named.

Prelims — what UPSC asks

• Sky colour without an atmosphere is UPSC's most repeated question from this chapter: astronaut's sky black/dark (1981, 1982), sky from the Moon black (1987), no atmosphere → black sky (1988), and a spaceship between Earth and Sun — sky jet black and stars not twinkling (1996). Learn the one-line reason: no air, no scattering.
• Twinkling and atmospheric optics: planets have apparent motion but do not twinkle (1981); the Sun's size and colour at dusk and dawn and the twinkle of stars as ‘optical illusions’ (2013). Rainbow: dispersion, refraction and internal reflection, all three (2013); rainbow by refraction vs thin-film colours by interference (1991). Scattering: ‘longer wavelengths scattered more’ is false (2002).
• The eye: retina = the film of a camera (1982); the pupil dilates in a dark room (1983); a ray diagram of myopia (1986); eye donation uses the cornea (1999, 2001); LASIK reshapes the cornea but is not for every age (2010).
• Likely next: myopia vs hypermetropia vs presbyopia statements with the right lens; power arithmetic for spectacles; why danger lights are red; why clouds are white; Tyndall effect examples; Raman effect and National Science Day (Beyond NCERT).

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter. Its use in GS3 is as the science line in applied answers: avoidable blindness and eye care (cataract surgery, spectacles for children, corneal transplants and eye donation), and atmospheric optics behind visibility and signal design (haze, fog, red warning lights).
• Answer frame: one-line science (with a labelled sketch — eye defect with its correcting lens, prism dispersion, or a raindrop) → technology or programme that uses it → benefits → limits → way forward.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.