Class 10 Science Chapter 10 — The Human Eye and the Colourful World (International Edition)

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NCERT / RBSE Syllabus 2026–27 · Learn by doing · Includes three labelled diagrams

Prepared by: NCERTClasses Team — textbook specialists and experienced teachers.
Based on: NCERT Class 10 Science textbook, Chapter 10, and the Board of Secondary Education, Rajasthan syllabus 2026–27.
Published: 24 September 2026 · Last updated: 24 September 2026

The convex lens from the last chapter is sitting inside your own eye right now — and unlike any glass lens, it can reshape itself.

And why does the sun look white overhead but turn red-orange at sunset? Why is the sky blue by day at all?

How to read this — first you think or try it, then comes the explanation:
🔍 Wonder · 🧪 Try it · 📖 Understand · 🔗 Apply · ✅ Check yourself
"Check yourself" has three levels — 1, 2, 3.

Contents

1. Foundation check
2. Structure of the human eye
3. Accommodation
4. Myopia (short-sightedness)
5. Hypermetropia (long-sightedness)
6. Refraction and dispersion through a prism
7. Atmospheric refraction
8. Scattering and the colour of the sky
9. Your ladder
10. Marking scheme
11. Common mistakes
12. Final test
13. Glossary
14. For parents and teachers
15. Beyond NCERT

Foundation check

1. What kind of image does a convex lens form? (Chapter 9)
2. What is the formula and unit for lens power?
3. What colours make up white light?

Answers

1. Depends on object position — both real and virtual images are possible. 2. P = 1/f (in metres), unit dioptre (D). 3. Seven colours — violet, indigo, blue, green, yellow, orange, red (VIBGYOR). This chapter examines each in detail.

1. Structure of the human eye

🔍 Wonder — A camera needs manual refocusing for near and far subjects. How does the eye do this instantly, with no buttons at all?

📖 Understand — Light first refracts at the cornea (the transparent bulge at the front) — most of the eye's refraction happens right here. It then passes through the pupil, whose size is controlled by the iris (small in bright light, large in dim light). Next, the eye lens — a convex lens — makes the fine adjustment needed to focus light onto the retina, where a real, inverted image forms. Retinal cells convert this into electrical signals sent via the optic nerve to the brain, which interprets it right-side up.

The fine-tuning of the lens's curvature is actually done by the ciliary muscles — a ring of muscle surrounding the lens. When viewing something near, they contract, making the lens thicker and more curved (shorter focal length); for distant objects, they relax, making the lens thinner (longer focal length).

🔗 Apply — Here's the difference from a camera: a camera lens physically slides back and forth to focus (changing v, as in Chapter 9). The eye's lens never moves from its position — instead, it changes its own curvature, changing its focal length.

The retina itself contains two kinds of light-sensitive cells: rod cells, which are highly sensitive in dim light but cannot distinguish colour, and cone cells, which need brighter light but are responsible for colour vision. This is exactly why colours are hard to make out at night or in a dimly lit room — in low light, mostly the colour-blind rod cells are doing the work.

✅ Check yourself

Level 1 Where does most of the eye's refraction happen? — Answer: At the cornea.
Level 2 What controls the size of the pupil? — Answer: The iris.

2. Accommodation

🧪 Try it — Bring your finger very close to your eye (around 5-6 cm) and try to focus on it. Think first — will it appear sharp?

What happened? Held that close, the finger blurs — no matter how hard the eye tries, it cannot focus that near. This shows accommodation has its own limits.

📖 Understand — The eye lens's ability to change curvature to focus on near or far objects is called accommodation. It has two limits:

Point	Meaning	Typical value (normal eye)
Near point	Closest distance at which vision stays sharp	About 25 cm
Far point	Farthest distance at which vision stays sharp	Infinity

🔗 Apply — A book is usually held at 25 cm or more precisely because that is the normal near point — closer than that puts extra strain on the eye, which over time causes fatigue.

✅ Check yourself

Level 1 What is the near point of a normal eye? — Answer: About 25 cm.
Level 2 Where is the far point of a normal eye? — Answer: At infinity.

3. Myopia (short-sightedness)

🔍 Wonder — Some people can't read a distant board clearly, yet read a book just fine up close — what exactly has gone wrong in the eye?

📖 Understand — In myopia, either the eyeball is too elongated or the lens's curvature is too great — so a distant object's image forms before it reaches the retina. The far point shrinks from infinity to some nearer distance.

Correction: a concave lens of the right focal length is used — it diverges the incoming rays slightly before they reach the eye, so they appear to come from the person's own (reduced) far point, letting the eye's own lens focus them correctly on the retina.

Worked example 1: A person cannot see objects clearly beyond 5 m. Find the type and power of the corrective lens needed.
Solution: The person's far point is 5 m. The corrective lens must form a virtual image of an object at infinity exactly at 5 m (the person's own far point). So u = −∞, v = −5 m. From the lens formula 1/v − 1/u = 1/f: 1/f = 1/(−5) − 0 = −1/5. f = −5 m, so P = 1/f = −0.2 D — a concave lens, power 0.2 dioptres.

✅ Check yourself

Level 1 Does the image form in front of or behind the retina in myopia? — Answer: In front (before the retina).
Level 2 Which lens corrects myopia? — Answer: A concave lens.
Level 3 If someone's far point is 2 m, what power of corrective lens is needed? — Answer: f = −2 m, so P = 1/(−2) = −0.5 D.

4. Hypermetropia (long-sightedness)

📖 Understand — Here, either the eyeball is too short or the lens's focal length is too long — so a nearby object's image tries to form behind the retina. The near point moves further than the normal 25 cm.

Correction: a convex lens of suitable power — it converges the incoming rays slightly before they reach the eye, letting the eye's own lens complete the job.

Worked example 2: A person's near point is 1 m (instead of the normal 25 cm). Find the power of the lens needed to read at the normal distance of 25 cm.
Solution: The object is at 25 cm (u = −0.25 m), but the eye can only see it if its image forms virtually at the person's own near point (1 m) — so v = −1 m. 1/f = 1/v − 1/u = 1/(−1) − 1/(−0.25) = −1 + 4 = 3. f = 1/3 m, so P = +3 D — a convex lens, power 3 dioptres.

✅ Check yourself

Level 1 Where does the image try to form in hypermetropia? — Answer: Behind the retina.
Level 2 Which lens corrects it? — Answer: A convex lens.
Level 3 Can the same person have both defects at once? — Answer: Yes — this is called presbyopia, where ageing itself reduces the eye's own accommodation ability; bifocal lenses (upper half concave-like for distance, lower half convex-like for near) are often used.

Point	Myopia (short-sightedness)	Hypermetropia (long-sightedness)
Problem	Distant objects blurry	Near objects blurry
Cause	Elongated eyeball or excessive lens curvature	Short eyeball or excessive focal length
Image forms	Before the retina	Behind the retina (tries to)
Corrective lens	Concave (negative power)	Convex (positive power)
Point affected	Far point shrinks nearer	Near point moves beyond 25 cm

5. Refraction and dispersion through a prism

🔍 Wonder — Where do a rainbow's seven colours come from, when sunlight itself looks white?

🧪 Try it — Without a prism, take an old CD/DVD and tilt its shiny surface in sunlight. Think first — will colours appear here too, and why?

What happened? A CD's surface has extremely fine, closely-spaced grooves that split light by a different principle (diffraction) — but the visual result is just as colourful, making prism-style dispersion easier to picture.

📖 Understand — White light is actually a mixture of seven colours — violet, indigo, blue, green, yellow, orange, red (VIBGYOR). When this light passes through a prism, each colour's different wavelength causes it to refract at a different angle — violet bends the most, red the least. This differential bending is what separates the colours — called dispersion.

🔗 Apply — A rainbow is essentially a giant natural prism effect made of millions of water droplets — each droplet refracts and partially internally reflects light, performing exactly this same dispersion.

✅ Check yourself

Level 1 Which colour bends the most in VIBGYOR? — Answer: Violet.
Level 2 Why does dispersion happen? — Answer: Because each colour in white light has a different wavelength, so each bends at a different angle through the prism.
Level 3 If a rectangular glass slab (from Chapter 9) were used instead of a prism, would white light split into colours just as clearly? — Answer: No, not clearly. A slab's two surfaces are parallel, so each colour that bends entering the slab bends back by almost the same amount leaving it — the colours separate slightly but largely recombine into something close to white again. A prism's angled, non-parallel surfaces are what prevent this recombination, keeping the colours visibly separated.

6. Atmospheric refraction

🔍 Wonder — Why do stars twinkle, while planets like Venus shine steadily?

📖 Understand — Earth's atmosphere isn't uniformly dense — air grows progressively denser from the upper atmosphere down to the ground. Starlight passing through this changing density is continuously, slightly refracted, and because the air is constantly in turbulent motion, this refraction keeps shifting moment to moment — causing the star's apparent brightness to flicker. This is twinkling. Planets are far closer to Earth and appear as small discs rather than single points — light from many points averages out the flicker almost entirely.

Another effect — the sun remains visible for a few minutes before actual sunrise and after actual sunset, because the atmosphere bends its rays down to us even when it's technically below the horizon. This is why the sun appears to rise early and set late — adding roughly 2 minutes to the apparent length of the day.

Worth noting — very near the horizon, the sun or moon can look flattened (wide but squashed vertically). This happens because light from the lower edge refracts slightly more than light from the upper edge (it travels a longer, more slanted path through denser air) — pulling the lower edge upward more, making the whole disc appear squashed.

Stars near the horizon can also appear to flicker with faint colour changes, not just brightness — this happens because the atmosphere refracts different wavelengths by very slightly different amounts (the same underlying idea as a prism's dispersion, just far more subtle), so a star's white light briefly separates into faint colour fringes as the turbulent air shifts.

✅ Check yourself

Level 1 Why do stars twinkle but planets don't? — Answer: Stars are distant point sources whose refraction shifts rapidly and visibly; planets, being closer, appear as small discs whose averaged light largely cancels the flicker.

7. Scattering and the colour of the sky

🔍 Wonder — In space, the sky looks black — but from Earth, it's blue. Why the difference?

🧪 Try it — Add a few drops of milk to a glass of clear water. Shine a torch from the side and view from the front, then look straight into the torch beam. Think first — will the colour look different from each angle?

What happened? Viewed from the side, the water looks faintly blue; looking straight into the beam gives a reddish-orange tint. The real sky behaves the same way.

📖 Understand — Molecules and small particles in the atmosphere scatter light in every direction — called scattering. This scattering is wavelength-dependent — short-wavelength blue light scatters far more than long-wavelength red light. So whenever we look at any part of the sky (away from the sun's direct direction) during the day, we see scattered blue light reaching us from all directions.

Why sunsets turn red — at sunset, the sun sits near the horizon, and its light must cross a much thicker slice of atmosphere to reach us. Over that long path, blue light gets almost entirely scattered away, leaving mostly the barely-scattered red-orange light to reach our eyes directly.

The Tyndall effect — when light passes through a colloidal medium (smoke-filled air, a dusty room, sunbeams filtering through forest trees), the particles scatter it so strongly that the entire path of the light becomes visible. This is the Tyndall effect.

🔗 Apply — Space has no atmosphere, so there's nothing to scatter light at all — that's exactly why astronauts see a black sky at all times, sunlight or not.

So why are clouds white, not blue? Air molecules are smaller than blue light's wavelength, so by Rayleigh's law they preferentially scatter blue. Cloud water droplets are far larger — large enough to scatter all colours of white light almost equally, with no preference for blue. All colours recombine into white — which is why clouds appear white (or grey when thick), never blue.

✅ Check yourself

Level 1 Which colour scatters the most? — Answer: Blue.
Level 2 Why does the setting sun look red? — Answer: Its light crosses a much thicker layer of atmosphere at that time, scattering away almost all the blue, leaving mainly red light to reach our eyes.
Level 3 Why do astronauts see a black sky even with the sun shining? — Answer: Scattering requires an atmosphere (gas molecules, particles). Space has none, so there's no scattering at all — only direct sunlight is visible if looking straight at the sun; everywhere else in the sky stays completely black.

Your ladder

Level	Total	If you got this many right	You are
Level 1	9	7+	Familiar
Level 2	9	6+	Proficient — board-exam ready
Level 3	4	3+	Skilled

Marking scheme

(An example of a typical examiner's approach — not an official scheme.)

Question (3 marks): What is myopia? Explain its cause and correction with a labelled ray diagram.

Definition and cause of the defect. 1 mark
Name of the corrective lens and how it works. 1 mark
Labelled ray diagram. 1 mark

Common mistakes

Common mistake	Correct fact
"Myopia needs a convex lens"	Myopia needs a concave lens; convex lens corrects hypermetropia.
"The eye lens slides to focus, like a camera"	The eye lens does not move — it changes its own curvature.
"All colours bend equally in a prism"	Each colour bends at a different angle — violet the most, red the least.
"The sky is blue because it reflects the ocean"	Wrong — the sky's blue colour comes from atmospheric scattering.
"The sky looks blue from space too"	No — with no atmosphere there, no scattering occurs; the sky appears black.

Final test — mixed

7+ correct = chapter mastered.

1. Which part of the eye does most of the refraction?
2. What is the near point of a normal eye?
3. Which lens corrects myopia?
4. Where does the image try to form in hypermetropia?
5. Which colour bends the least in dispersion?
6. Why do stars twinkle?
7. Why is the sky blue?
8. (Chapter 9) State the lens formula.
9. (Chapter 8) What determines a child's sex?

Answers — 1. The cornea | 2. About 25 cm | 3. Concave lens | 4. Behind the retina | 5. Red | 6. Because of continuous refraction through the atmosphere's changing density | 7. Because blue light is scattered the most | 8. 1/v − 1/u = 1/f | 9. The sex chromosome (X or Y) carried by the fertilising sperm

Glossary

English	Hindi	English	Hindi
Accommodation	समंजन क्षमता	Dispersion	विक्षेपण
Near point	निकट बिंदु	Atmospheric refraction	वायुमंडलीय अपवर्तन
Far point	दूर बिंदु	Scattering	प्रकीर्णन
Myopia	निकट दृष्टि दोष	Tyndall effect	टिंडल प्रभाव
Hypermetropia	दूर दृष्टि दोष	Retina	रेटिना
Prism	प्रिज्म	Cornea	कॉर्निया

For parents and teachers

Five-minute check: Ask the difference between myopia and hypermetropia (which lens, where the image forms). Ask why the sky is blue and sunsets are red. Check if VIBGYOR order is remembered.

To try at home: the milk-drop-in-water-plus-torch experiment (scattering), and viewing colours off an old CD — both completely safe and clearly visible. Health note: this lesson is educational only — see an eye specialist for any real vision concern.

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Beyond NCERT — For Curious Readers

This section goes beyond the RBSE/NCERT Class 10 syllabus and is not required for the board exam. It draws only on government and internationally reputed university sources, for curious readers who want to see where this chapter leads.

How the brain flips the image right-side up

This chapter explains that the retinal image is always inverted, yet we never perceive the world upside down. Neuroscience attributes this to the visual cortex, which processes the inverted signal from the eye and constructs an upright perceptual experience — this is the brain's interpretive work, not anything the eye itself does. In the 1890s, psychologist George Stratton wore special image-inverting goggles for days at a stretch, and found that after some days his brain adapted and began perceiving the world as upright again — a striking demonstration of how adaptable the brain's interpretation of raw visual signals really is.

Newton's famous prism experiment

Around 1666, Isaac Newton let a beam of sunlight enter a darkened room through a small hole and placed a glass prism in its path. He not only observed white light splitting into seven colours, but also showed that a second prism could recombine those seven colours back into white light — proving that colours are not "created" by the prism but were already present within white light all along.

A common myth — the "bigger" moon near the horizon

Many people believe the moon or sun looks "bigger" near the horizon because of atmospheric refraction. Astronomers consider this a misconception — the moon's actual angular size stays essentially identical whether it's near the horizon or overhead, easily verified by measuring it in photographs. The real cause is psychological, known as the "moon illusion," where the brain judges the moon's size relative to nearby objects like trees and buildings, making it seem larger — the exact refraction-based explanation for this illusion remains genuinely disputed among researchers. It's worth keeping this separate from the chapter's actual, well-established atmospheric refraction facts (twinkling stars, the early sunrise/late sunset effect).

Eye health in India

India's National Programme for Control of Blindness and Visual Impairment runs free cataract surgery and vision-screening camps nationwide. Uncorrected refractive errors — exactly the kind this chapter describes — are recognised globally as a leading, and fully correctable, cause of childhood vision impairment.

Why the sky isn't always blue — Mars as a counterexample

Earth's blue daytime sky results from the specific composition of our atmosphere. According to imagery from NASA's Mars rover missions, the Martian sky appears pale pinkish-brown by day, because fine dust particles suspended in its thin atmosphere scatter light in a completely different way from Earth's atmosphere — direct proof that a planet's sky colour depends on its own particular atmosphere, not on any universal rule.

Beyond glasses — contact lenses and laser correction

The concave and convex lenses this chapter describes don't have to sit in a frame. A contact lens performs exactly the same optical correction, curved to sit directly on the cornea instead. A more permanent option, laser refractive surgery (commonly known by the brand-associated term LASIK), reshapes the cornea's own curvature directly using a precisely controlled laser, changing the eye's natural refractive power so that, ideally, no external lens is needed at all afterward — an elegant, modern extension of the same core idea: correcting how strongly the eye bends incoming light.

Sources for this section

• ncert.nic.in — NCERT, Government of India
• ePathshala — Ministry of Education, Government of India
• mohfw.gov.in — Ministry of Health and Family Welfare, Government of India

Note: nothing from these sources has been copied into this text; all statements above are written in the author's own words for general educational understanding, and are not medical or professional advice.

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Continue reading
← Previous: Chapter 9 — Light: Reflection and Refraction (8 marks)
→ Next: Chapter 11 — Electricity (7 marks)

Related material

• Chapter 9 — Light: Reflection and Refraction (English)
• Chapter 10 — मानव नेत्र तथा रंग-बिरंगा संसार (Hindi)
• Class 10 Science Mega Test — 200 MCQs

Based on the Class 10 Science (Code 07) syllabus 2026–2027 of the Board of Secondary Education, Rajasthan, Ajmer — Chapter 10, marks weightage 4. Textbook: Science, NCERT. All diagrams are original.