Human Eye and Colourful World NCERT Solution, study material, CBSE Notes

NCERT Solution: Human Eye and Colourful World

Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours of the spectrum obtained.

Order of colours from bottom to top: Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Is the position of a star as seen by us its true position? Justify your answer.

The density of atmospheric layers increases as we move from top to bottom. Due to this, starlight bends towards the normal as it passes through different layers of atmosphere. Due to this, the apparent position of star is a little above its actual position in sky

Why do we see a rainbow in the sky only after rainfall?

Formation of rainbow is only possible when dispersion of light takes place through a suitable surface. After rainfall, some raindrops remain in the clouds. Moreover, the opposite side of sky works like a screen on which rainbow is formed. Hence, rainbow is seen in the sky only after rainfall.

Why is the colour of the clear sky blue?

Out of all the colours in the visible spectrum, blue colour scatters the most. Due to this, it is the blue colour which reaches our eyes. As a result, the colour of sky appears blue.

What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

The sky appears reddish during sunrise/sunset but it appears white at noon. During noon, the sunlight has to travel less distance to reach us. Most of the colours reaching us get scattered. Due to this, Sky appears white at noon. Colours near the red end of the spectrum scatter the least. During sunset, and sunrise, sunlight needs to travel more distance to reach us. Red colour is able to reach us because it is scattered the least. Hence, sky appears reddish during sunrise/ sunset.

Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?

The human eye has following main parts:

Cornea: Human eye is spherical in shape. It has tough white coat which protects the interior of the eye. The front portion of this coat is transparent and is called cornea.
Iris: This is a dark muscular structure behind the cornea. Unique colour of a person�s eye is because of colour of iris.
Pupil: The small opening in the iris is called pupil. Iris controls the size of the pupil and thus controls the amount of light entering the eye. Light enters the eye through pupil.
Lens: Lens is thicker at the middle and is made of transparent material. Lens focuses the light on the back of the eye; called retina.
Retina: The back of the eye is called retina. It works like a screen; on which image is formed. There are numerous light-sensitive nerve cells on retina. These nerve cells are connected to optic nerve.

Formation of Image in Eye: Light rays enter the eye through pupil and pass through lens. Lens focuses light rays on retina. Real, inverted and smaller image is formed on retina. Optic nerve carries the message to the brain. The brain interprets the message and we get the sense of vision.

When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?

When a person is unable to clearly see distant objects, he is considered a myopic person. Such a person is suffering from myopia.
When a person is unable to clearly see a nearby object, he is considered a hypermetropic person. Such a person is suffering from hypermetropia.

Correction of Myopia: A person suffering from myopia needs to use a concave lens of suitable focal length. The concave lens diverges the rays coming from infinity. After refraction from the concave lens, the rays appear to be coming from the far point of this person’s eye. Due to this, a clear image of distant object is made on the retina of that person. That is how a myopic person is able to clearly see distant objects; with the help of suitable concave lens.

Correction of Hypermetropia: A hypermetropic person needs to use a convex lens of suitable focal length. The convex lens converges the light rays coming from a nearby object. As a result, these light rays appear to be coming from the near point of this person’s eyes. Due to this, a clear image of nearby object is made on the retina of that person. That is how a hypermetropic person is able to clearly see nearby objects; with the help of suitable convex lens.

Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.

• ABC is prism with base BC
• PE is incident ray on surface AB. It makes < PEN with the normal NE. This angle is angle of incidence.
• After entering the prism, the light ray bends towards normal. In this case, EF is refracted ray. <N' EF is angle of refraction.
• Once the refracted ray emerges from prism into air, it bends away from normal. In this firgure, FS is emergent ray. <SFM is angle of emergence.
Angle of Deviation: The angle between incident ray and emergent ray is called angle of deviation. Here, <SGH is angle of deviation or <D.