The Human Eye and the Colourful World | Class 10 Science (NCERT Ch 10)

CBSE Class 10 › Natural Phenomena

The Human Eye & Colourful World

NCERT Chapter 10 • Vision, Prisms, and Atmospheric Effects

NCERT 2025–26 Optics Natural Phenomena

Illustration showing the human eye structure and a prism dispersing light.

While the previous chapter dealt with light and lenses, this chapter explores how the human eye uses light to see and explains fascinating natural phenomena like rainbows, twinkling stars, and the blue sky.

1. The Human Eye

The human eye works like a camera. It forms an image on a light-sensitive screen called the retina.

Anatomy of the Human Eye showing Cornea, Iris, Pupil, Lens, and Retina. — Structure of the Human Eye.

Key Parts and Functions

Part	Function
Cornea	Thin membrane through which light enters. Most refraction occurs here.
Iris	Dark muscular diaphragm that controls the size of the pupil.
Pupil	Regulates the amount of light entering the eye.
Crystalline Lens	Provides finer adjustment of focal length to focus objects on the retina.
Retina	Light-sensitive screen with cells that generate electrical signals.
Optic Nerve	Transmits visual signals to the brain.

2. Power of Accommodation

The ability of the eye lens to adjust its focal length is called accommodation. This is achieved by the Ciliary Muscles.

Distant Objects: Muscles relax → Lens becomes thin → Focal length increases.
Nearby Objects: Muscles contract → Lens becomes thick → Focal length decreases.

Important Limits:

Near Point (Least Distance of Distinct Vision): The minimum distance to see distinct objects without strain. For a normal adult, it is 25 cm.
Far Point: The farthest point the eye can see clearly. For a normal eye, it is Infinity.

3. Defects of Vision and Their Correction

Sometimes the eye loses its power of accommodation, resulting in blurred vision. The three common refractive defects are:

Ray diagrams showing Myopia, Hypermetropia, and their correction with lenses. — Correction of Myopia with Concave lens and Hypermetropia with Convex lens.

A. Myopia (Near-sightedness)

A person can see nearby objects clearly but cannot see distant objects distinctly.

Cause: Excessive curvature of the eye lens or elongation of the eyeball.
Result: Image forms in front of the retina.
Correction: Concave Lens (Diverging lens).

B. Hypermetropia (Far-sightedness)

A person can see distant objects clearly but cannot see nearby objects distinctly.

Cause: Focal length of the eye lens is too long or eyeball has become too small.
Result: Image forms behind the retina.
Correction: Convex Lens (Converging lens).

C. Presbyopia

An age-related defect where the power of accommodation decreases. The near point recedes.

Cause: Weakening of ciliary muscles and diminishing flexibility of the eye lens.
Correction: Bi-focal lenses (Upper part Concave for distance, Lower part Convex for reading).

Q1. What is meant by power of accommodation of the eye?

The ability of the eye lens to adjust its focal length to see both nearby and distant objects clearly is called the power of accommodation.

Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

Since the person cannot see distant objects (Myopia), they need a Concave Lens (diverging lens) to correct the vision.

Q3. What is the far point and near point of the human eye with normal vision?

For a normal eye:
Near Point: 25 cm.
Far Point: Infinity.

Q4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

The student can see nearby (notebook) but not far away (blackboard). This is Myopia (near-sightedness). It can be corrected using a Concave Lens.

4. Refraction of Light Through a Prism

A triangular glass prism has two triangular bases and three rectangular lateral surfaces. The angle between two lateral faces is the Angle of Prism (A).

Unlike a glass slab, the emergent ray in a prism is not parallel to the incident ray. It bends towards the base. The angle between the incident ray and the emergent ray is called the Angle of Deviation (D).

5. Dispersion of White Light

Dispersion is the splitting of white light into its component colors.

Spectrum: The band of colored components (VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Orange, Red).
Cause: Different colors bend at different angles. Red bends the least, Violet bends the most.

Dispersion of white light through a prism and formation of a rainbow. — Dispersion through a prism and Rainbow formation in water droplets.

Rainbow Formation

A rainbow is a natural spectrum caused by dispersion of sunlight by tiny water droplets. The droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again.

6. Atmospheric Refraction

The earth’s atmosphere is not uniform. When light enters the atmosphere, it undergoes refraction due to varying optical densities.

Twinkling of Stars

Stars are point-sized sources of light. As starlight passes through the atmosphere, its path varies due to air turbulence. The apparent position of the star fluctuates, and the amount of light entering the eye flickers, causing twinkling.

Note: Planets do not twinkle because they are extended sources; the variations average out.

Advance Sunrise and Delayed Sunset

Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset.

7. Scattering of Light

The scattering of light involves the deflection of light by minute particles.

Tyndall Effect

When a beam of light strikes fine particles (like smoke or dust), the path of the beam becomes visible. This is the Tyndall effect.

Why is the Sky Blue?

The molecules of air are smaller than the wavelength of visible light. They scatter shorter wavelengths (Blue) more strongly than longer wavelengths (Red). This scattered blue light enters our eyes.

Fact: If Earth had no atmosphere, the sky would look dark (black) because there would be no scattering.

Colour of the Sun at Sunrise and Sunset

At sunrise/sunset, sunlight travels a longer distance through the atmosphere. Blue light is scattered away, leaving mostly red light (longer wavelength) to reach our eyes.

8. Chapter Exercises

Practice these NCERT exercise questions to master the chapter:

Q1. 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

(b) Accommodation.

Q2. The human eye forms the image of an object at its
(a) cornea (b) iris (c) pupil (d) retina

(d) Retina.

Q3. 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

Q4. 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

Q5. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

(i) Distant Vision:
$P = -5.5 \text{ D}$ .
$f = \frac{1}{P} = \frac{1}{-5.5} = -0.18 \text{ m}$ (approx) or $-18 \text{ cm}$ .

(ii) Near Vision:
$P = +1.5 \text{ D}$ .
$f = \frac{1}{P} = \frac{1}{+1.5} = +0.67 \text{ m}$ or $+66.7 \text{ cm}$ .

Q6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

To see distant objects (at infinity), the image must form at the far point (80 cm).
Object distance $u = \infty$ . Image distance $v = -80 \text{ cm}$ (in front of lens).
Using Lens Formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{-80} - \frac{1}{\infty} = -\frac{1}{80}$ .
$f = -80 \text{ cm} = -0.8 \text{ m}$ .
Power $P = \frac{1}{f} = \frac{1}{-0.8} = -1.25 \text{ D}$ .
Nature: Concave Lens.

Q7. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

The lens must create a virtual image at 1 m (-100 cm) for an object placed at 25 cm (-25 cm).
$u = -25 \text{ cm}, v = -100 \text{ cm}$ .
$\frac{1}{f} = \frac{1}{-100} - \frac{1}{-25} = \frac{-1 + 4}{100} = \frac{3}{100}$ .
$f = \frac{100}{3} \text{ cm} = \frac{1}{3} \text{ m}$ .
Power $P = \frac{1}{f (m)} = \frac{1}{1/3} = +3.0 \text{ D}$ .
Nature: Convex Lens.

Q8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

The ciliary muscles cannot contract beyond a certain limit to increase the curvature of the lens. Therefore, the focal length cannot be decreased below a minimum limit to focus objects closer than 25 cm.

Q9. What happens to the image distance in the eye when we increase the distance of an object from the eye?

The image distance in the eye remains constant (distance between lens and retina). The eye adjusts the focal length of the lens (accommodation) to keep the image focused on the retina.

Q10. Why do stars twinkle?

Stars twinkle due to atmospheric refraction. As starlight passes through the atmosphere, the refractive index of air layers fluctuates. This changes the apparent position of the star and the amount of light entering the eye, causing the twinkling effect.

Q11. Explain why the planets do not twinkle.

Planets are much closer to Earth and act as extended sources of light (a collection of points). The fluctuations in light from different points average out to zero, nullifying the twinkling effect.

Q12. Why does the sky appear dark instead of blue to an astronaut?

At very high altitudes (space), there is no atmosphere to scatter light. Without scattering, no light reaches the eye from the sky, making it appear dark.

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