# NCERT Science Class 10 Chapter 11 the Human Eye and the Colourful World CBSE Board Sample Problems Short Answer

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## Question

A person needs a lens of power -5.5 dioptre for correcting his distinct vision. For correcting his near vision he needs a lens +1.5 dioptre. What is the focal length of the lens required for correcting (j) distinct vision, and (ii) near vision?

### Solution

(î) Power of lens needed for correction distant vision of the person (P)

Focal length of lens required for correcting distant vision (f) .

(ii) For correcting near vision the power of lens required (P)

Focal length of lens required for correcting near vision (f) .

## Question

What is presbyopia? State the causes of this defect? How is presbyopia of a person corrected?

### Solution

Presbyopia is the defect of human eye in which a person is unable to see the nearby as well as far off objects clearly.

Causes:

(1) Decrease is the power of accommodation of the eye due to ageing

(2) Weakening of the ciliary muscles It can be corrected by using a bifocal lens (upper half concave and the lower half convex)

## Question

The near point of hypermetropic eye ¡s loo cm. What is the nature and power of the lens required to enable him to read a book placed at 25 cm from the eyes?

### Solution

Given,

Near point of hypermetropic eye = 100cm

That means the image of an eye can be seen at 100cm.

Object is at 25cm, which is needed to be seen clear.

So,

So, corrective lens of focal length (f) is needed to show object at 100cm in front of eye.

By the lens formula.

Near point = 100 cm

Object distance

cm (convex lens in case of hype rmetropia)

Using lens formula

We get cm

## Question

Why does it takes some time to see objects in a cinema hail when we just enter the hail from bright sunlight? Explain in brief.

### Solution

The pupil regulates and controls the amount of light entering the eye.

In bright sunlight, the size of the pupil ¡s small and when we enter the cinema hail ¡t takes some time for the pupil to expand ¡n size due to dim light.

## Question

Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an observer on the moon? Justify your answer with a reason.

### Solution

• Early in the morning, the sun is near the horizon, sunlight reaches us after covering a longer distance through thick layers of atmosphere. So the most of the blue light and other tight of shorter wavelengths are scattered away by the particles in the atmosphere.

• The light that reaches us is of longer wavelengths such as red light thus giving a reddish appearance. This phenomenon will not be observed by an observer on the moon because of the absence of atmosphere on the moon.

## Question

Describe an activity to show that the colours of white light splitted by a glass prism can be recombined to get white light by another identical glass prism. Also draw ray diagram to show the recombination of the spectrum of white tight.

### Solution

When a glass prism is used to obtain a spectrum of sunlight and a second identical prism in an inverted position with respect to the first position then it will allow all the colours of spectrum to recombine. Thereby a beam of white light will emerge from the other side of the second prism.

## Question

Give reasons:

(a) Stars twinkle at night

(b) Sun appears red in the morning.

### Solution

(a) The twinkling of star is due to atmospheric refraction, the star light will undergo refraction continuously before it reaches the earth.

(b) Early in the morning sun is at horizon which is at more distance than in noon.

Sunlight coming from horizon has to pass through thicker layer of atmosphere and is dispersed through larger particles suspended in atmosphere. Larger particles will scatter red light more as it has maximum wavelength. So, it appears red.

## Question

List two causes of presbyopia. How it can be corrected?

### Solution

Causes:

(i) Gradual weakinng of ciliary musles

(ii) Dimishing flexibility of eye lens.

This defect can be corrected by using a bifocal lens of suitable focal length.

## Question

1. What is dispersion of white light? Name the various colours of spectrum of white light in proper sequence.

2. Why are danger signals red in colour?

### Solution

1. The splitting of white light into seven colours is called dispersion. Seven colours of spectrum are violet, indigo, blue, green, yellow, orange and red which is known as

‘VI BGYOR’.

2. Wavelength of red colour is more. So, it is least scattered and can be seen from large distance.

## Question

Draw course of rays through (i) A glass slab (ii) A glass prism. Why net deviation through a glass slab is zero, but net deviation through a glass prism is not zero? (3)

### Solution

• Two refractions through a rectangular glass slab, Light emerges from rectangular glass slab in a direction parallel to that in which it entered the glass slab. However, the ray is slightly displaced laterally.

(1)

• A triangular glass prism has two triangular bases and three rectangular lateral surfaces.

• These surfaces are inclined to each other at some suitable angle, which is angle of prism.

• Thus, deviation of a ray on passing through a prism, nature of material of prism and angle of incidence of the ray on one face of the prism. (1)

• Net deviation in glass slab is zero because deviation produced at first refraction is equal and opposite to the deviation produced at second refraction. In case of prism, the deviations produced in the two refraction’s add up, being in the same direction. (1)

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