CBSE Class 10 Physics The Human Eye and The Colorful World Worksheet Set A

Question. A very thin narrow beam of white light is made incident on one face of rectangular glass slab and one face of the combination of two prisms, one up and one down in contact. Comment on the nature of the behaviour of the emergent beam in the above two cases.
Answer: Case 1: The emergent beam refracted through a rectangular glass slab emerges parallel to the incident ray and is shifted sideward slightly. No dispersion occurs.
Case 2: The emergent beam from the second inverted identical prism is again white light and emerges parallel to the incident beam and shifted sideward slightly. Overall no dispersion occurs due to the combination of two prisms placed inverted to each other.
Thus emergent ray in both the cases emerges from the opposite parallel faces and is parallel to the incident ray.

Question. 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?
Answer: The far point of this myopic person is 80 cm. (This means that this person can see a distant object kept at infinity clearly if the image of this object is formed at his far point.)
u = ∞, v = –80 cm, f = ?
Using Lens formula
1/v - 1/u = 1/f          ∴   1/-18 - 1/∞ = 1/f
⇒ 1/f = -1/80  - 0 = -1/80       ∴   f = –80 cm = –0.8 m
Negative sign indicates the lens is concave.
Power, P = 1/f = 1/- 0.8 m = –1.25            ∴   P = –1.25 D
A concave lens of power –1.25 D is required to correct the problem.

Question. What eye defect is myopia? Describe with a neat diagram how this defect of vision can be corrected by using a suitable lens. 
Answer: Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he cannot see the far objects clearly.
Causes of myopia. Myopia is caused
— due to the elongation of the eye ball.
— due to decrease in the focal length of the eye lens. The eye lens becomes more convergent.
Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.

Question. What is a spectrum? Why do different coloured rays deviate differently on passing through a glass prism? 
Answer: Meaning of spectrum. When a beam of white light is passed through a prism, a band of seven colours is formed on a white screen. This is called spectrum of white light.
The dispersion of white light occurs because the angle of refraction or angle of bending of different components of the white light is different when passing through the glass prism. When white light consisting of seven colours falls on a prism, each colour is refracted or deviated by a different angle, with the result that the seven coloured band, i.e., spectrum is formed.

Question. What eye defect is hypermetropia? Describe with a ray diagram how this defect of vision can be corrected by using an appropriate lens. 
Answer: Long sightedness is hypermetropia. Due to this defect, a person is not able to see the nearby objects clearly but can see the distant objects clearly.
Causes of long-sightedness. It is caused due to the following reasons:
— Normal increase in the focal length of the eye lens. The lens becomes less convergent.
— Shortening of the eyeball size.
Long sightedness can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.
When a convex lens of suitable power is placed in front of the hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at a point near to the hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.

Question. What is the colour of the clear sky during day-time? Give reason for it. 
Answer: During day time the colour of the sky is blue.
Reason: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So these particles scatter more effectively the light rays of shorter wavelength at the blue end than light of longer wavelengths at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky.

Question. State the difference in colours of the Sun observed during sunrise/sunset and noon. Give explanation for each. 
Answer: The Sun and surrounding sky appear red at sunrise and at sunset because at this time the Sun is near the horizon and sunlight has to travel the greatest distance through the atmosphere to reach us. Thus most of the blue colour present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes. When the Sun is overhead (as at noon) then the light coming from the Sun has to travel a relatively shorter distance through the atmosphere to reach us. Thus only a little of blue colour of the white light is scattered. Since the light coming from the overhead Sun has almost all its component colours in the right proportion,
therefore the Sun in the sky overhead appears white.

Question. When and where do we see a rainbow? How is a rainbow formed? Draw a labelled diagram to illustrate the formation of a rainbow. 
Answer: Rainbow is caused by dispersion of sunlight by tiny water droplets present in the atmosphere. The water droplets act like small prisms. They refract the incident sunlight and then reflect it internally and finally refract it again when it comes out of the rain-drop. A rainbow is always formed in the direction opposite to that of the Sun.

Question. Explain giving reason why the sky appears blue to an observer from the surface of the earth? What will the colour of the sky be for an astronaut staying in the international space station orbiting the earth?
Answer: Sky appears blue. The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. So these particles scatter more effectively the light rays of shorter wavelength at the blue end than light of longer wavelengths at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky. Colour of sky to an astronaut. There is no atmosphere containing air in space to scatter sunlight. As there is no scattering of light in space, the scattered light does not reach the eyes and the sky appears dark instead of blue to an astronaut in outer space.

Question. Explain with help of a labelled diagram, the cause of twinkling of stars. 
Answer: Twinkling of Stars. The twinkling of a star is due to atmospheric refraction of star light. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends star light towards the normal, the apparent position of the star is slightly different from the actual position.
This apparent position of the star is not stationary, but keeps on changing slightly as the physical conditions of the earth’s atmosphere are not stationary.
Since the stars are very distant, they act as point-sized sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers—the star sometimes appears brighter and at other times appears fainter, which is the twinkling effect.

Answer : Different wavelengths deviate differently in the prism because the angle of refraction for different colours having different wavelengths is different while passing through the glass prism (medium). A light ray is refracted when it passes from one medium to another at an angle and its speed changes. At the interface, it is bent in one direction if the material it enters is denser (when light slows down) and in the other direction if the material is less dense (when light speeds up). Because different wavelengths (colours) of light travel through a medium at different speeds, the amount of bending is different for different wavelengths. Violet is bent the most and red the least because violet light has a shorter wavelength, and short wavelengths travel more slowly through a medium than longer ones do.

Corrected eye: Concave (or any divergent) lens diverges the parallel rays from infinity to an extent that they appear to diverge from F. They get focused at retina.

Causes of defect in defective eye: The defect arise due to either :

Due to either or both the causes, the eye is not able to focus the rays from distant object at retina. Focusing is there at a point O in front of retina. Image formed on retina is blurred.

The defective eye is however able to focus the object upto its far point.

Correct of defect: We have observed that the defective eye has not been able to focus less inclined rays but has been able to focus more inclined rays.

To correct this defect, a concave or a divergent lens of appropriate focal length is placed in front of the eye so that parallel rays coming from infinity appear, after refraction through the lens, to come from the far point when eye is unaccommodated.

These parallel rays from a distant object shall be brought to focus upon the retina.

Obviously, the focal length of such a lens is equal to the distance of the far point from the eye.

i. the length of eyeball of eye lens has decreased.

ii. the focal length of eye lens has decreased.