## The Human Eye and The Colourful World Exemplar

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Multiple Choice Questions:

Q-1: A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power

If a person cannot see distantly, object kept beyond 2 m then he is suffering from myopia.

Solution: (b)

As the person has the eye defect, myopia, therefore, a concave lens has to be used whose focal length will be f = – 2 m (using sign convention). Thus,

Q-2: A student sitting on the last bench can read the letters, written on the blackboard but is not able to read the letters written in his next book. Which of the following statements is correct?

(a) The near point of his eyes has receded away

(b) The near point of his eyes has come closer to him

(c) The far point of his eyes has come closer to him

(d) The far point of his eyes has receded away

Hypermetropia may have blurred vision to a person when looking at objects close to them and clearer vision when looking at objects in the distance.

Solution: (a)

The student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook because he is suffering from hypermetropia or farsightedness. He can see distant objects clearly but cannot see nearby objects distinctly.

Q-3: A prism ABC (with BC as a base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in the figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?

Solution:

Generally, in case of a prism (II), the formation of spectrum is shown below

In the above figure, from top the third colour is yellow. But we can see that from bottom the third colour is blue which is the colour of sky. So, we can obtain the correct situation by inverting the prism. Thus the required orientations can be bound in case II.

Q-4: At noon the sun appears white as

(a) light is least scattered

(b) all the colours of the white light are scattered away

(c) blue colour is scattered the most

(d) red colour is scattered the most

Solution: (a)

At noon the sun appears white because the light from the sun is directly over head and travel relatively shorter distance. The sun appears white as only a little of the blue and violet colours are scattered.

Q-5: Which of the following phenomena of light are involved in the formation of a rainbow?

(a) Reflection, refraction and dispersion

(b) Refraction, dispersion and total internal reflection

(c) Refraction, dispersion and internal reflection

(d) Dispersion, scattering and total internal reflection

Solution: (c)

A rainbow is caused by dispersion, refraction and internal reflection of sunlight by tiny water droplets, present in the atmosphere and always formed in a direction opposite to that of the sun. The water droplets act like small prisms. They refract and dispense the incident sunlight, then reflect it internally and finally and finally refract it again when it comes out of the raindrop.

Q-6: Twinkling of stars is due to atmospheric

(a) dispersion of light by water droplets

(b) refraction of light by different layers of varying refractive indices

(c) scattering of light by dust particles

(d) internal reflection of light by clouds

Solution: (b)

The twinkling of a star is due to atmospheric refraction of lights of stars. These lights on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth.

The path of rays of light coming from the distant star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers. The star sometimes appears brighter and at some other time, fainter gives us the twinkling effect.

Q-7: The clear sky appears blue, because

(a) blue light gets absorbed in the atmosphere

(b) ultraviolet radiations are absorbed in the atmosphere

(c) violet and blue lights get scattered more than lights of all other colours by the atmosphere

(d) light of all other colours is scattered more than the violet and blue colour lights by the atmosphere

Solution: (c)

The clear sky appears blue is due to Rayleigh scattering of sunlight. The molecules in the air scatter blue light from the sun more than they scatter red light.

Q-8: Which of the following statements is correct regarding the propagation of light of different colours of white light in air?

(a) Red light moves fastest

(b) Blue light moves faster than green light

(c) All the colours of the white light move with the same speed

(d) Yellow light moves with the mean speed as that of the red and the violet light

Solution: (c)

The propagation of light of different colours of white light in air or vacuum move with the same speed but different wavelengths and frequencies.

Q-9: The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light

(a) is scattered the most by smoke or fog

(b) is scattered the least by smoke or god

(c) is absorbed the most by smoke or fog

(d) moves fastest in air

Solution: (b)

The danger signals installed at the top of tall buildings are red in colour because among all other colours, red colour is scattered the least by smoke or fog. This is primarily because wavelength of red colour is the largest. Thus it can be easily seen from a distance.

Q-10: Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?

(a) Dispersion of light                                                       (b) Scattering of light

(c) Total internal reflection of light                                    (d) Reflection of light from the earth

Solution: (b)

The reddish appearance of the sun at sunrise or sunset is due to least scattering of red light. The red light have the maximum wavelength. The rays of light have to travel a larger part of the atmosphere.

Q-11: The bluish colour of water in deep sea is due to

(a) The presence of algae and other plants found in water

(b) reflection of sky in water

(c) scattering of light

(d) absorption of light by the sea

Solution: (c)

The bluish colour of water in deep sea is due to scattering of light as the very fine particles in water scatter mainly blue light. The red, green, orange and yellow having longer wavelengths are absorbed more strongly by water than blue which has shorter wavelength. So, when white light from the sun enters the sea, it is mostly the blue that gets returned.

Q-12: When light rays enter the eye, most of the refraction occurs at the

(a) crystalline lens                                                       (b) outer surface of the cornea

(c) iris                                                                          (d) pupil

Solution: (b)

When the light rays enters the eye through a thin membrane, forms the transparent bulge on the front surface of the eyeball, called the cornea. Most of the refraction for the light rays entering the eye occurs at this outer surface of the comes.

Q-13: The focal length of the eye lens increase when eye muscles

(a) are relaxed and lens becomes thinner

(b) contract and lens becomes thicker

(c) are relaxed and lens becomes thicker

(d) contract and lens becomes thinner

Solution: (a)

The focal length of the eye lens increases when eye muscles relaxed and becomes thinner. The sharp image of the distant object is formed at the retina. This enables us to focus accurately on distant objects.

Q-14: Which of the following statements is correct?

(a) A person with myopia can see distant objects clearly

(b) A person with hypermetropia can see nearby objects clearly

(c) A person with myopia can see nearby object clearly

(d) A person with hypermetropia cannot see distant objects clearly

Solution: (c)

A person with myopia can see nearby objects clearly. White a person with hypermetropia can see distant objects clearly.