NCERT Exemplar Solutions
Class 10 - Science - CHAPTER 11: The Human Eye and the Colourful World

Long Answer Questions

Structure of the Human Eye:

The human eye is a spherical organ approximately 2.5 cm in diameter. Its major parts are:

1. Cornea: The transparent, curved front surface of the eye. It refracts most of the incoming light.
2. Aqueous Humour: A transparent fluid behind the cornea that maintains its curvature and refractive power.
3. Iris: The coloured part of the eye. It controls the amount of light entering the eye by adjusting the size of the pupil.
4. Pupil: The opening in the centre of the iris. It regulates the intensity of light entering the lens.
5. Eye Lens: A transparent, convex, flexible lens that focuses light on the retina. Its curvature is controlled by ciliary muscles.
6. Ciliary Muscles: These muscles adjust the focal length of the lens to focus on objects at various distances.
7. Vitreous Humour: A transparent gel between the lens and retina that maintains the shape of the eye.
8. Retina: A light-sensitive inner surface containing rods and cones. It forms real and inverted images.
9. Optic Nerve: It carries visual information from the retina to the brain.

Functioning of the Eye:

Light entering the eye passes through the cornea, aqueous humour, pupil, lens, and vitreous humour. The lens focuses the light rays on the retina, where an image is formed. The optic nerve transmits the information to the brain, which interprets it as an upright image.

Seeing Nearby and Distant Objects—Accommodation:

The ability of the eye lens to change its focal length so that both near and far objects can be seen clearly is called accommodation.

1. For distant objects: The ciliary muscles relax, the lens becomes thin, and focal length increases.
2. For nearby objects: The ciliary muscles contract, the lens becomes thicker, and focal length decreases.

Thus, the human eye adjusts its focal length to clearly see objects at various distances.

Myopia (Near-sightedness):

A person is said to be myopic when he can see near objects clearly but cannot see distant objects distinctly. This happens because either the eye lens becomes too curved or the eyeball becomes elongated, causing the image of distant objects to form in front of the retina.

Correction: Myopia is corrected using a concave lens of suitable power. The concave lens diverges incoming light rays so that the image shifts back onto the retina.

Hypermetropia (Far-sightedness):

A person is hypermetropic when he can see distant objects clearly but cannot see nearby objects distinctly. This happens when the eyeball is too short or the eye lens becomes less curved, causing the image of nearby objects to form behind the retina.

Correction: Hypermetropia is corrected using a convex lens of suitable power. The convex lens converges the light rays before they enter the eye so that the image is formed on the retina.

Ray Diagrams:

Diagrams include:

• Image formation in a myopic eye and its correction using a concave lens.
• Image formation in a hypermetropic eye and its correction using a convex lens.

Refraction Through a Glass Prism:

When a narrow beam of white light enters a triangular glass prism, it bends towards the normal at the first refracting surface due to its higher optical density. Inside the prism, the light travels towards the second refracting surface, where it bends again but away from the normal as it emerges into air.

Ray Path Description:

1. The incident ray strikes surface AB of the prism at an angle of incidence \( i \).
2. It refracts inside the prism and bends towards the normal.
3. At the second surface AC, the refracted ray emerges into the air, bending away from the normal at an angle of emergence \( e \).
4. The emergent ray is deviated from the original path by an angle \( D \).

Angle of Deviation (\( D \)):

The angle between the direction of the incident ray and the emergent ray is called the angle of deviation. It is given by:

\[ D = (i + e) - A \]

where \( A \) is the angle of the prism.

Reddish Appearance at Sunrise and Sunset:

During sunrise and sunset, the Sun is near the horizon. The sunlight must travel a much longer distance through the Earth's atmosphere. Due to this long path, most of the shorter wavelengths such as blue and violet are scattered away by atmospheric particles.

The longer wavelengths like red and orange scatter least, so they reach the observer. Hence, the Sun appears reddish.

Why the Sun Does Not Appear Red at Noon:

At noon, the Sun is overhead and sunlight travels a much shorter distance through the atmosphere. Little scattering occurs, and all colours reach the observer almost equally. Thus, the Sun appears white or yellowish rather than red.

Dispersion of Light:

Dispersion is the phenomenon of splitting of white light into its constituent colours (Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a prism.

Cause of Dispersion:

Different colours of light have different wavelengths and therefore different refractive indices for the prism material. Violet light deviates the most, while red deviates the least.

Process:

1. A narrow beam of white light is incident on one face of a prism.
2. The light refracts inside the prism and splits into seven colours due to different deviations.
3. The emergent beam displays a spectrum on a screen.

Ray Diagram:

Diagram shows a beam of white light entering a prism, emerging separated into VIBGYOR colours.

Atmospheric Refraction:

Atmospheric refraction occurs because the Earth's atmosphere consists of layers of air with varying densities and refractive indices. Light rays from celestial objects bend gradually as they pass through these layers of different optical densities.

Twinkling of Stars:

Stars appear as point sources of light. Due to atmospheric turbulence, the refractive index of the layers keeps changing. This causes the path and intensity of star light to vary continuously, making the star appear to twinkle.

Why Planets Do Not Twinkle:

Planets appear larger because they are much closer to Earth. They act as extended sources of light. The variations in brightness due to atmospheric refraction average out over their larger apparent size, so planets do not twinkle.