The Human Eye and the Colourful World Class 10 Notes | CBSE Chapter 10 Science

The Human Eye and the Colourful World is Chapter 10 of CBSE Class 10 Science. This chapter explains how the human eye works as a natural optical instrument, the common defects of vision and how spectacles correct them, and the everyday optical wonders around us - rainbows, the blue sky, and the reddish sun at sunrise and sunset.

By the end of these notes you will be able to label the eye, draw the ray diagrams for myopia and hypermetropia, and confidently explain dispersion, atmospheric refraction and scattering in any board answer. The chapter carries roughly 3–5 marks in the CBSE board, and the diagram-based questions here are among the easiest marks to secure if you have practised the figures.

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Table of Contents

• Key Concepts - structure of the eye, accommodation, defects of vision, dispersion, atmospheric refraction, scattering
• Weightage in Board & Entrance Exams
• Important Definitions
• Solved Examples
• Common Mistakes to Avoid
• Important Questions for Board Exams
• Quick Revision Points

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Weightage in Board & Entrance Exams

Exam	Typical Weightage	Most-Tested Areas
CBSE Board (Class 10)	3–5 marks	Labelled eye diagram, myopia & hypermetropia ray diagrams, why the sky is blue / sun is red
Foundation / NTSE	1–2 questions	Power of accommodation, near point, lens power for a defect
Reasoning / assertion	High frequency	Twinkling of stars, advance sunrise, Tyndall effect, dispersion order

The single most repeated 3-mark question is a ray diagram of a defect and its correction, so master the two figures below. The single most repeated 1-mark reasoning question is “why does the sky appear blue” or “why does the sun look red at sunset”.

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Key Concepts

1. The Human Eye

The human eye is a natural optical instrument that works much like a camera. Its lens system forms a real, inverted image on a light-sensitive screen called the retina, and the brain turns that image the right way up for us.

Structure of the Human Eye

Part	Function
Cornea	Transparent front surface; most of the refraction (bending) of light entering the eye happens here
Aqueous humour	Clear watery fluid between the cornea and the lens; helps refract light and keeps the cornea in shape
Iris	Coloured, muscular part of the eye; controls the size of the pupil
Pupil	Dark opening in the middle of the iris; regulates the amount of light entering - it widens (dilates) in dim light and narrows (constricts) in bright light
Ciliary muscles	Hold the eye lens and change its curvature (shape) so it can focus on near or far objects
Crystalline lens	Transparent, flexible, convex lens behind the pupil; fine-focuses light onto the retina by changing its thickness
Retina	Light-sensitive screen at the back of the eye; contains rod cells (sense dim light) and cone cells (sense colour and bright light)
Vitreous humour	Transparent jelly-like substance filling the large space between the lens and the retina
Optic nerve	Carries the electrical impulses from the retina to the brain, which interprets them as the image we “see”

How the Eye Forms an Image

1. Light enters through the cornea, where most of the bending (refraction) takes place.
2. It passes through the pupil, whose size is controlled by the iris.
3. The crystalline lens further refracts the light and forms a sharp image on the retina.
4. The retina converts the light into electrical nerve impulses.
5. The optic nerve carries these impulses to the brain, which interprets them as the scene we perceive.

The image formed on the retina is real and inverted; the brain automatically flips it so that we see the world upright.

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2. Power of Accommodation

The ability of the eye lens to adjust its focal length so that it can focus on objects at different distances is called the power of accommodation. It is the ciliary muscles that make this adjustment happen.

• Looking at distant objects: the ciliary muscles relax, the lens becomes thin, its focal length increases, and distant objects are focused on the retina.
• Looking at nearby objects: the ciliary muscles contract, the lens becomes thick and more curved, its focal length decreases, and nearby objects are focused on the retina.

The focal length of the eye lens cannot be reduced below a certain limit. That is why we cannot read a book comfortably if it is held too close to the eye.

Near Point and Far Point

Term	Definition	For a normal eye
Near point (least distance of distinct vision)	Closest distance at which the eye can see an object clearly and comfortably without strain	25 cm (denoted by D)
Far point	Farthest distance at which the eye can see an object clearly	Infinity (∞)

So a normal, healthy eye can see everything clearly between 25 cm and infinity. When either of these limits shifts, the eye has a defect of vision.

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3. Defects of Vision and Their Correction

The three common defects of vision are myopia, hypermetropia and presbyopia. Each is corrected using a spectacle lens of a suitable type and power. Remember that the power of a lens is P = 1/f (with f in metres, measured in dioptre, D); a concave lens has a negative power and a convex lens has a positive power.

a) Myopia (Short-sightedness / Near-sightedness)

Problem: the person can see nearby objects clearly but distant objects appear blurred.

Cause:

• the eyeball has become too long (elongated), or
• the eye lens is too thick, giving it too much converging power.

What happens: the image of a distant object forms in front of the retina instead of on it, so it looks blurred. The far point has moved from infinity to a nearer point.

Correction: a concave lens (diverging lens) of suitable negative power. It diverges the parallel rays slightly before they enter the eye, so the eye lens then focuses them exactly on the retina.

b) Hypermetropia (Long-sightedness / Far-sightedness)

Problem: the person can see distant objects clearly but nearby objects appear blurred.

Cause:

• the eyeball has become too short, or
• the eye lens is too thin, giving it too little converging power.

What happens: the image of a nearby object forms behind the retina. The near point has moved farther than the normal 25 cm.

Correction: a convex lens (converging lens) of suitable positive power. It adds the extra converging power the eye is missing, so the image forms on the retina.

c) Presbyopia

Problem: the eye gradually loses its power of accommodation with age, so both nearby and distant objects become hard to focus.

Cause: the ciliary muscles weaken and the eye lens loses its flexibility, so the near point recedes.

Correction: bifocal lenses, whose upper part is a concave lens (for distant vision) and lower part is a convex lens (for near vision). A person with both myopia and hypermetropia also uses bifocal lenses.

Comparison of Myopia and Hypermetropia

Feature	Myopia	Hypermetropia
Can see clearly	Near objects	Far objects
Cannot see clearly	Far objects	Near objects
Image forms	In front of the retina	Behind the retina
Cause (eyeball)	Too long / lens too thick	Too short / lens too thin
Lens used to correct	Concave (diverging)	Convex (converging)
Sign of lens power	Negative (−)	Positive (+)

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4. Refraction of Light Through a Prism and Dispersion

When a ray of light passes through a glass prism, it bends towards the base of the prism at both surfaces. The angle between the incident ray and the emergent ray is called the angle of deviation.

When white light passes through a prism it does more than bend - it splits into its component colours. This splitting of white light into its seven colours is called the dispersion of light, and the band of colours obtained is called a spectrum.

The spectrum of white light is VIBGYOR:

• Violet, Indigo, Blue, Green, Yellow, Orange, Red.

Why does dispersion happen? White light is a mixture of seven colours, and each colour has a different wavelength. The glass bends each colour by a different amount, so they emerge along slightly different paths:

• Violet has the shortest wavelength, so it slows down the most in glass and bends (deviates) the most.
• Red has the longest wavelength, so it bends the least.

Recombination of the spectrum: Isaac Newton showed that if a second, identical prism is placed inverted after the first, the seven colours recombine to give back white light. This proves that white light is genuinely made of these seven colours.

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5. Atmospheric Refraction

The atmosphere is made of layers of air at different temperatures and densities. As light passes through these constantly changing layers, it bends by tiny amounts - this continuous bending of light by the atmosphere is called atmospheric refraction.

Phenomena Caused by Atmospheric Refraction

• Twinkling of stars: a star is so far away that it acts as a point source of light. Its light passes through many air layers of changing density, so the amount of refraction keeps changing. This makes the star’s apparent position and brightness flicker - we see it as twinkling.
• Why planets do not twinkle: planets are much closer and act as a collection of many point sources (an extended source). The flickering from different points averages out, so a planet’s light appears steady.
• Stars appear slightly higher near the horizon: refraction bends starlight coming from near the horizon, so the star appears a little higher than its true position.
• Advance sunrise and delayed sunset: the sun is visible about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset. Light from the sun (below the horizon) is refracted downward by the atmosphere, making the sun appear raised above the horizon. This gives us roughly 4 extra minutes of daylight each day. For the same reason the sun appears flattened (oval) at sunrise and sunset.

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6. Scattering of Light

When a beam of light strikes tiny particles, the particles send the light off in many directions. This spreading of light by particles in its path is called scattering of light. The colour of the scattered light depends on the size of the particles.

Key rule (Rayleigh scattering): very fine particles scatter mainly shorter wavelengths. So blue and violet light are scattered much more than red and orange light. In fact, the amount of scattering is inversely proportional to the fourth power of the wavelength (1/λ⁴).

Why is the Sky Blue?

Sunlight contains all seven colours. When it enters the atmosphere, the tiny air molecules (mainly N₂ and O₂) scatter the shorter wavelengths (blue) far more than the longer ones (red). This scattered blue light reaches our eyes from every part of the sky, so the clear sky looks blue. (Violet is scattered even more, but our eyes are less sensitive to violet and some of it is absorbed high up, so we perceive blue.)

Why is the Sky Black to an Astronaut?

High above the atmosphere there are almost no molecules to scatter sunlight. With no scattering, no light reaches the astronaut’s eyes from the sides, so the sky (space) appears black even though the sun is shining.

Why Does the Sun Appear Reddish at Sunrise and Sunset?

At sunrise and sunset the sunlight has to travel through a much greater thickness of atmosphere to reach us than it does at noon. Along this long path most of the blue and shorter wavelengths are scattered away, so mainly the red and orange light (which is scattered least) reaches our eyes. That is why the sun and the sky near the horizon look reddish.

Why Does the Sun Appear White at Noon?

At noon the sun is overhead and its light travels through the least thickness of atmosphere. Very little scattering happens, so all colours reach us in nearly equal amounts and the sun appears white (or slightly yellowish).

Tyndall Effect

The Tyndall effect is the scattering of light by colloidal particles (particles larger than molecules but too small to see) that makes the path of a light beam visible. Everyday examples:

• a beam of sunlight becomes visible when it enters a dusty room through a small hole;
• the beam of a car’s headlights is visible in fog or mist;
• the smoke rising from a mosquito coil looks bluish because the fine particles scatter blue light.

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7. Formation of a Rainbow

A rainbow is a natural spectrum that appears in the sky after rain. It is caused by the dispersion of sunlight by tiny water droplets suspended in the atmosphere, which act as tiny prisms.

How it forms: sunlight enters a water droplet and refracts (bending and dispersing into seven colours); the light then reflects internally off the back of the droplet; finally it refracts again as it leaves the droplet and reaches the observer. So a rainbow is a combination of refraction, dispersion and total internal reflection.

A rainbow is always seen:

• in the direction opposite to the sun (with the sun behind the observer);
• after a rain shower, or near a waterfall or a fountain of spray;
• as a semicircular arc, with red on the outer edge and violet on the inner edge.

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Important Definitions

Term	Definition
Power of accommodation	Ability of the eye lens to change its focal length to focus on objects at different distances
Near point	Closest distance at which the eye can focus clearly (25 cm for a normal eye)
Far point	Farthest distance at which the eye can see clearly (infinity for a normal eye)
Least distance of distinct vision	Same as the near point - the minimum distance for comfortable clear vision (25 cm)
Myopia	Defect in which distant objects are blurred; the image forms in front of the retina
Hypermetropia	Defect in which nearby objects are blurred; the image forms behind the retina
Presbyopia	Age-related loss of accommodation; difficulty in seeing both near and far objects
Dispersion of light	Splitting of white light into its seven component colours by a prism
Spectrum	The band of seven colours (VIBGYOR) obtained by dispersion of white light
Atmospheric refraction	Bending of light as it passes through air layers of changing density
Scattering of light	Spreading of light in different directions by tiny particles in its path
Tyndall effect	Scattering of light by colloidal particles that makes the path of a beam visible

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Solved Examples

Example 1

Q. A person cannot see objects clearly beyond 50 cm. Name the defect, the lens needed and its power.

A. The far point has shifted from infinity to 50 cm, so the defect is myopia. A concave lens is required, which must form a virtual image of a very distant object at the person’s far point (50 cm).

Using the lens formula 1/v − 1/u = 1/f with u = −∞ and v = −50 cm gives 1/f = −1/50, so f = −50 cm = −0.5 m.

Power P = 1/f = 1/(−0.5) = −2 D (a concave lens of power −2 dioptre).

Example 2

Q. Why do stars twinkle but planets do not?

A. A star is extremely far away and behaves as a point source. Its light is refracted continuously by the atmosphere’s changing layers, so its apparent brightness and position keep varying - we see twinkling. A planet is much closer and behaves as an extended source (many points); the variations average out, so a planet shines steadily.

Example 3

Q. Why is the clear sky blue?

A. Air molecules are much smaller than the wavelength of visible light. By Rayleigh’s law the scattering is proportional to 1/λ⁴, so short-wavelength blue light is scattered about ten times more than red. This scattered blue light reaches our eyes from all over the sky, making it look blue.

Example 4

Q. A person needs a lens of power +2.5 D for correction. Which defect do they have and what is the focal length?

A. A positive power means a convex lens, so the defect is hypermetropia. Focal length f = 1/P = 1/2.5 = 0.4 m = 40 cm.

Example 5

Q. Why does the sun appear red at sunrise and sunset but white at noon?

A. At sunrise and sunset sunlight travels through a much thicker layer of atmosphere; most of the blue light is scattered away, so mainly red light reaches us and the sun looks red. At noon the light travels through the least atmosphere, very little is scattered, and the sun appears white.

Example 6

Q. To an astronaut in space, does the sky look blue or black? Give the reason.

A. It looks black. There is no atmosphere (no molecules) in space to scatter sunlight, so no scattered light reaches the astronaut from the sides, and the sky appears dark even in bright sunlight.

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Common Mistakes to Avoid

• Do not swap the lenses: myopia (can’t see far) uses a concave lens; hypermetropia (can’t see near) uses a convex lens.
• In myopia the image forms in front of the retina; in hypermetropia it forms behind the retina - label this clearly in ray diagrams.
• The image on the retina is real and inverted, not virtual and upright.
• Twinkling of stars is due to atmospheric refraction, while the blue sky and red sun are due to scattering - do not mix the two.
• Violet bends the most and red the least; students often reverse this.
• Write the spectrum order correctly as VIBGYOR, and remember the near point is 25 cm (not 25 m).

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Important Questions for Board Exams

1-Mark Questions (VSA)

1. What is the least distance of distinct vision for a normal human eye?
2. Which type of lens is used to correct myopia?
3. Why does the sun appear reddish at sunrise?
4. State the function of the iris in the human eye.
5. Name the phenomenon responsible for the blue colour of the clear sky.

2–3-Mark Questions (SA)

1. What is the power of accommodation of the eye? How do the ciliary muscles bring it about?
2. Distinguish between myopia and hypermetropia on the basis of cause, image position and correction.
3. What is dispersion of light? Draw a diagram to show dispersion of white light by a glass prism.
4. Why do stars twinkle while planets do not?
5. What is the Tyndall effect? Give two everyday examples.
6. Explain, with a ray diagram, how myopia is corrected.

5-Mark Questions (LA)

1. Draw a labelled diagram of the human eye and explain how it forms an image and adjusts to see near and distant objects.
2. Describe the common defects of vision, giving the cause and the correction (with ray diagrams) for each.
3. Explain atmospheric refraction and use it to account for the advance sunrise and the delayed sunset.
4. Explain the scattering of light. Why is the sky blue, why is the sun red at sunset, and why is the sky black to an astronaut?

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Quick Revision Points

• Path of light in the eye: cornea (maximum refraction) → pupil (controls light) → lens (fine focus) → retina (real, inverted image) → optic nerve → brain.
• Power of accommodation: the eye lens changes its focal length via the ciliary muscles to focus near and far.
• Near point = 25 cm; far point = infinity for a normal eye.
• Myopia: cannot see far; image in front of retina; corrected by a concave lens (negative power).
• Hypermetropia: cannot see near; image behind retina; corrected by a convex lens (positive power).
• Presbyopia: age-related; both near and far affected; corrected by bifocal lenses.
• Dispersion: white light → VIBGYOR through a prism; violet bends most, red bends least.
• Blue sky: air molecules scatter short-wavelength blue light most (Rayleigh scattering, 1/λ⁴).
• Red sun at sunrise/sunset: long atmospheric path scatters blue away, leaving red.
• Sky is black to an astronaut: no atmosphere, so no scattering.
• Stars twinkle (point source, atmospheric refraction); planets do not (extended source).
• Rainbow: sunlight + water droplets → refraction + dispersion + internal reflection.