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Class 10 Human Eye and the Colourful World notes: NCERT Science Chapter 10 revision

CMDLearn · 4 November 2026

These are short revision notes for "The Human Eye and the Colourful World", Chapter 10 of the NCERT Class 10 Science textbook (Reprint 2026-27), pages 161 to 170. The chapter has two halves: the eye as a lens system with its three common defects, and what light does in a prism, in raindrops and in the air. These notes give a one-page summary table, then go section by section, and end with the key words, the diagram to practise, the common mistakes and 8 quick questions of our own with worked answers.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts this chapter in Unit III, Natural Phenomena, worth 12 of the 80 theory marks, shared with Light – Reflection and Refraction (curriculum, PDF page 4). For this chapter the unit lists how the lens works in the human eye, the defects of vision and how they are corrected, refraction through a prism, dispersion and scattering of light and their uses in daily life. It leaves out the Sun's colour at sunrise and sunset (PDF page 6). More on the units: CBSE Class 10 Science syllabus 2026-27.

The chapter on one page

1. The human eye (section 10.1, pages 161-162)

• The eye behaves much like a camera. Its lenses focus an image on the retina, a layer at the back that responds to light (page 161).
• The eyeball is roughly a sphere, about 2.3 cm across (page 161).
• Light comes in through the cornea, the clear bulge at the front. Most of the bending of light happens where it first meets the cornea, at its outer surface; the eye lens only fine-tunes the focal length (page 161).
• The iris, a dark muscular diaphragm behind the cornea, sets the size of the pupil, and the pupil controls how much light gets in (pages 161-162).
• The lens forms a real, inverted image on the retina. Its light-sensitive cells fire electrical signals, which travel along the optic nerves to the brain, and the brain makes sense of them (page 162).

2. Accommodation, near point and far point (section 10.1.1, page 162)

• The eye lens is made of a fibrous, jelly-like material, and the ciliary muscles change its curvature (page 162).
• Looking far away: the muscles relax, the lens gets thinner and its focal length gets longer.
• Looking at something close: the muscles contract, the lens gets more curved and thicker, and its focal length gets shorter.
• Accommodation is this ability of the lens to change its focal length. It has a limit: the focal length cannot go below a certain minimum (page 162).
• Near point (least distance of distinct vision): the closest an object can be and still be seen clearly without strain, about 25 cm for a young adult. Far point: the farthest point seen clearly, infinity for a normal eye. So a normal eye sees clearly from 25 cm to infinity (page 162).
• Cataract: in old age the lens can turn milky and cloudy, causing partial or full loss of sight; surgery can bring vision back (page 162).

3. Defects of vision and their correction (section 10.2, pages 162-164)

• A bi-focal lens has a concave upper part for distant vision and a convex lower part for near vision (page 164).
• Contact lenses and surgery can also correct these defects (page 164).
• Eye donation (box, pages 164-165): a donor can be of any age or sex, and people who wear spectacles or have had cataract surgery can still donate. The eyes must be taken out within 4-6 hours of death, so inform the nearest eye bank at once. One pair can restore sight to up to four people with corneal blindness.

4. Refraction through a prism (section 10.3, pages 165-166)

• A triangular glass prism has two triangular bases and three rectangular side faces; the angle of the prism is the angle between two of its side (lateral) faces (page 165).
• At the first face light goes from air into glass and bends towards the normal; at the second face it goes from glass into air and bends away from the normal (page 166).
• Unlike a glass slab, where the ray comes out parallel to its original direction, the prism's shape turns the emergent ray at an angle to the incident ray. That angle is the angle of deviation (pages 165-166).
• In Fig. 10.4: PE is the incident ray, EF the refracted ray and FS the emergent ray; i, r and e are the angles of incidence, refraction and emergence; A is the angle of the prism and D the angle of deviation (page 166).

5. Dispersion and the rainbow (section 10.4, page 167)

• A prism splits white light into a band of colours, the spectrum: violet, indigo, blue, green, yellow, orange, red (VIBGYOR). This splitting is dispersion (page 167).
• It happens because each colour bends through a different angle: red the least, violet the most (page 167).
• Newton placed a second, identical prism upside down after the first and got white light back. This suggested that sunlight is made of seven colours. Any light giving a spectrum like sunlight's is called white light (page 167).
• Rainbow: tiny water droplets act like small prisms. Inside each drop, sunlight is refracted and dispersed on the way in, reflected internally, then refracted again on the way out. A rainbow always forms in the direction opposite the Sun; you can also see one in the spray of a waterfall or fountain with the Sun behind you (page 167).

6. Atmospheric refraction (section 10.5, page 168)

• Air just above a fire is hotter, lighter and has a slightly lower refractive index than the cooler air above it. Because the air keeps moving, objects seen through it seem to waver (page 168).
• Twinkling of stars: starlight is bent again and again as it passes through layers of air of changing refractive index. A star near the horizon looks a little higher than it really is, and this apparent position keeps shifting, so the light reaching the eye flickers. Stars are so far away that they act as point sources (page 168).
• Planets do not twinkle: they are much closer, so they are extended sources; the flicker from their many points averages out (page 168).
• Early sunrise, late sunset: the Sun is seen about 2 minutes before it actually crosses the horizon and about 2 minutes after it sets. The flattened look of the Sun's disc at these times has the same cause (page 168).

7. Scattering of light (section 10.6, page 169)

• Tyndall effect: fine particles in the air (smoke, tiny droplets, dust, air molecules) send light off in all directions, so a beam's path becomes visible, as when a thin sunbeam enters a smoky room or sunlight passes through misty forest (page 169).
• The colour scattered depends on particle size: very fine particles scatter mostly blue, larger ones scatter longer wavelengths, and large enough ones can make the scattered light look white (page 169).
• Blue sky: air molecules are smaller than the wavelength of visible light, so they scatter the blue end more strongly than the red end. Red light's wavelength is about 1.8 times that of blue. Without an atmosphere the sky would look dark, as it does to passengers flying very high (page 169).
• Danger signals are red because fog and smoke scatter red the least, so it keeps its colour over a distance (page 169).

Key words at a glance

• Accommodation: the eye lens changing its focal length to focus near and far objects.
• Near point: the closest distance of clear, strain-free vision; about 25 cm normally.
• Far point: the farthest distance of clear vision; infinity for a normal eye.
• Myopia / hypermetropia / presbyopia: short sight / long sight / the near point receding with age.
• Angle of deviation: the angle between the incident ray and the emergent ray of a prism.
• Spectrum: the band of colours a beam of light splits into.
• Dispersion: the splitting of white light into its colours.
• Atmospheric refraction: bending of light by layers of air of different refractive index.
• Tyndall effect: a beam's path made visible by scattering from fine particles.

The diagram to know: myopia and its correction

Board papers often ask for the eye-defect diagrams. Practise this three-part one (it follows Fig. 10.2):

1. (a) Far point of a myopic eye: draw the eyeball as a circle with the lens near the front. Mark a point O a short way in front of it. Rays from O enter the eye and meet on the retina: O is the farthest point this eye sees sharply.
2. (b) Myopic eye: draw parallel rays from a very distant object. They meet at a point in front of the retina and have spread out again by the time they reach it, so the image is blurred.
3. (c) Correction: put a concave lens in front of the eye. The parallel rays leave it spreading out, as if they started at O (show this with dotted lines back to O). The eye can focus light from O, so the rays now meet on the retina.
4. Label the three parts, and write "concave lens of suitable power" under (c).

For hypermetropia (Fig. 10.3), mark N (the defective near point) and N′ (the normal near point, 25 cm). Rays from an object at N′ meet behind the retina; a convex lens bends them so they seem to come from N, and the image lands on the retina.

Mistakes that cost marks

• Swapping the lenses. Myopia: concave (diverging). Hypermetropia: convex (converging). A memory aid: the myopic eye already bends too much, so the lens must spread the light out.
• Saying the eye lens does most of the bending. The cornea does most of it; the lens fine-tunes (page 161).
• Getting accommodation backwards. Near object: muscles contract, lens thicker, focal length shorter. Far object: the opposite.
• Saying violet bends least. Red bends least, violet most.
• Looking for a rainbow towards the Sun. It is always on the opposite side.
• Power in centimetres. P = 1/f needs f in metres: f = −50 cm gives P = 1/(−0.5) = −2 D.

Quick check: 8 questions of our own

These are our own questions, written for these notes and checked against the textbook pages named; no practising teacher has reviewed them yet. Every number was re-computed by a script.

Question 1. (a) Roughly what shape and size is the human eyeball? (b) Where does most of the bending of the light happen? (c) After the image forms, what carries the information to the brain?

Answer: (a) nearly a sphere, about 2.3 cm across; (b) at the cornea, on its outer surface; (c) electrical signals along the optic nerves. (a) The book gives the eyeball as approximately spherical with a diameter of about 2.3 cm (page 161). (b) The book says the cornea's outer surface does most of the refraction; the lens only adjusts the focal length (page 161). (c) The retina's light-sensitive cells produce electrical signals that the optic nerves take to the brain (page 162).

Question 2. Use a simple model eye in which the lens sits a fixed 2.0 cm from the retina, so every sharp image has v = +2.0 cm. (a) What focal length does the lens need for a star (object at infinity)? (b) What focal length does it need for a page 25 cm away? (c) Which focal length is shorter, and what do the ciliary muscles do to get it? (d) Find the two powers and their difference.

Answer: (a) 2.0 cm; (b) 50/27 ≈ 1.85 cm; (c) the one for the page: the muscles contract; (d) +50 D and +54 D, a difference of 4 D. (a) With u at infinity, 1/u = 0, so 1/f = 1/v and f = 2.0 cm (lens formula, Chapter 9). (b) u = −25 cm: 1/f = 1/v − 1/u = 1/2 + 1/25 = 27/50, so f = 50/27 ≈ 1.85 cm. (c) To focus a near object the ciliary muscles contract, the lens becomes thicker and its focal length decreases (page 162). (d) P = 1/f in metres: 1/0.02 = 50 D and 27/0.5 = 54 D, so the eye adds 4 D. (The real eye is not this simple, since the cornea does most of the bending; the model only shows why the focal length must change.)

Question 3. Name the defect and the lens that corrects it for each person. (a) Kabir reads his book easily but cannot read the number on a bus until it is a few metres away. (b) Mrs Rao, 58, could read at 25 cm when young; now she holds the newspaper well beyond that, though she sees distant hills clearly. Her doctor says the cause is age. (c) Mr Singh would rather not wear spectacles at all. Name two other ways of correcting a refractive defect that the chapter mentions.

Answer: (a) myopia, concave lens; (b) presbyopia, reading glasses with a convex lens; (c) contact lenses, or surgery. (a) Clear near vision with a far point short of infinity is myopia; a concave lens of suitable power corrects it (page 163). (b) A near point that moves away with age, from weaker ciliary muscles and a less flexible lens, is presbyopia, and she needs corrective glasses to see near things (pages 163-164). Near vision is helped by a convex lens: it is the near-vision (lower) part of a bi-focal (page 164). (c) Besides spectacles, the book says refractive defects can now be corrected with contact lenses or through surgical interventions (page 164).

Question 4. An optometrist gives Meera distance glasses of power −0.5 D. (a) Is the lens concave or convex? (b) What is its focal length in cm? (c) For a distant object, this lens forms a virtual image at its focus, which is where Meera's far point must be. How far can she see clearly without glasses? (d) Name her defect.

Answer: (a) concave; (b) −200 cm; (c) about 2 m; (d) myopia. (a) Negative power means a diverging, concave lens (Chapter 9, page 158). (b) f = 1/P = 1/(−0.5) = −2 m = −200 cm. (c) For u at infinity, 1/v = 1/f, so v = −200 cm: the image sits 2 m in front of the eye, at her far point. (d) A far point nearer than infinity, corrected by a concave lens, is myopia (page 163).

Question 5. Arjun sees distant hills clearly, but the print of a book held at 25 cm looks blurred. His reading glasses are marked +2.5 D. (a) Name the defect. (b) Is the lens converging or diverging, and what is its focal length in cm? (c) How does Arjun's near point compare with the 25 cm of a normal young eye? (d) Why would these glasses not help someone who cannot see distant objects clearly?

Answer: (a) hypermetropia; (b) converging (convex), f = +40 cm; (c) it is farther than 25 cm; (d) that person has myopia, which needs a diverging (concave) lens. (a) Seeing far objects clearly but not near ones is hypermetropia, or far-sightedness (page 163). (b) Positive power means a converging, convex lens (Chapter 9, page 158): f = 1/P = 1/2.5 m = 0.4 m = 40 cm, and a convex lens is what the book prescribes for this defect (page 163). (c) For hypermetropia the near point is farther away than the normal near point of 25 cm (page 163). (d) Not seeing distant objects is myopia, which is corrected with a concave lens of suitable power, not a convex one (page 163).

Question 6. In a diagram, a ray KL strikes face AB of a triangular glass prism ABC at L, travels through the glass along LM, and leaves through face AC at M along MN. (a) Which of KL, LM and MN is the incident ray, which is the refracted ray, and which is the emergent ray? (b) At L the ray goes from air into glass, and at M from glass into air. At each point, does it bend towards the normal or away from it? (c) If the same ray were sent through a rectangular glass slab instead, how would the ray coming out be directed compared with the ray going in?

Answer: (a) KL incident, LM refracted, MN emergent; (b) towards the normal at L, away from it at M; (c) parallel to the ray going in, only shifted sideways. (a) KL strikes the first face, so it is the incident ray; LM travels inside the glass, so it is the refracted ray; MN leaves the second face, so it is the emergent ray. They match PE, EF and FS in the book's figure (page 166). (b) Light passing into a denser medium, air to glass, bends towards the normal at the first face; passing out into a rarer medium, glass to air, it bends away from the normal at the second face (page 166). (c) A slab's two refracting faces are parallel, so the ray coming out runs parallel to the ray going in, just shifted a little to one side (page 165).

Question 7. (a) At 5 pm, with the Sun low in the west, a shower passes. Which part of the sky should you watch for a rainbow? (b) At 8 am you water the garden with a hose spray. Where should the Sun be for you to see a rainbow in the spray? (c) Put in order what happens to sunlight in a raindrop: internal reflection; refraction and dispersion on entering; refraction on leaving.

Answer: (a) the east; (b) behind you; (c) refraction and dispersion on entering, internal reflection, refraction on leaving. (a) A rainbow always forms opposite the Sun, so with the Sun in the west look east (page 167). (b) The book notes rainbows in waterfall or fountain spray when the Sun is behind the viewer (page 167). (c) Droplets refract and disperse the light, reflect it inside, then refract it again as it leaves (page 167).

Question 8. Name the effect behind each: (a) the beam from a cinema projector shows up in the dusty air of the hall; (b) clouds, made of fairly large water droplets, look white; (c) the far side of a hot tandoor seems to shimmer when you look across the top of it; (d) a star low over the horizon looks slightly higher than its real position.

Answer: (a) scattering (Tyndall effect); (b) scattering by large particles; (c) atmospheric refraction; (d) atmospheric refraction. (a) Fine particles in the air scatter the light, making the beam's path visible (page 169). (b) When scattering particles are large enough, the scattered light can look white (page 169). (c) The hot air above the tandoor has a slightly lower refractive index than the cooler air higher up and keeps moving, so the apparent position wavers (page 168). (d) The atmosphere bends starlight towards the normal, so the star appears above its true position (page 168).

Revise with questions

Test yourself next: Class 10 Human Eye and the Colourful World, 8 practice questions with answers. Free practice on this chapter, marked instantly in your browser: Class 10 Human Eye and the Colourful World questions. The chapter before this one: Class 10 Light, 8 practice questions with answers. The other notes in this set: Electricity, Acids, Bases and Salts and Life Processes. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

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Sources

All checked on 4 November 2026. NCERT page numbers are the printed page numbers of the textbook (printed page = PDF page + 160).

• NCERT, Science, Textbook for Class X, "The Human Eye and the Colourful World" (Chapter 10, Reprint 2026-27), pages 161-170: section 10.1 and accommodation (pages 161-162); 10.2, defects of vision and the eye-donation box (pages 162-165); 10.3, the prism (pages 165-166); 10.4, dispersion and the rainbow (page 167); 10.5, atmospheric refraction (page 168); 10.6, scattering (page 169). ncert.nic.in/textbook/pdf/jesc110.pdf
• CBSE, Science curriculum, Class X, 2026-27: unit marks (PDF page 4) and Unit III contents for this chapter (PDF page 6): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, section numbers and figure numbers are from the documents above. The wording of these notes, the diagram steps, the list of mistakes and the 8 questions are our own.

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