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

Short Answer Questions

Question. A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.
(i) Write the name and cause of the phenomenon observed.
(ii) Where else in nature is this phenomenon observed?
(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light.
Answer: (i) The phenomenon of splitting of white light into seven colours on passing through a glass-prism is called dispersion of light. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism.
(ii) Formation of rainbow is an example of dispersion of white light in nature.
(iii) The formation of spectrum of seven colours shows that white light is made up of lights of seven different colours mixed together. That is, white light is a mixture of seven colours (or seven coloured lights), i.e., red, orange, yellow, green, blue, indigo and violet.

Question. With the help of a labelled diagram, explain why the sun appears reddish at the sunrise and the sunset.
Answer: At the time of sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. During the long journey of sunlight, most of the shorter wavelength blue-colour present in it is scattered out and away from our line of sight so, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red colour due to which the sun appears red. Due to the same reason, the sky surrounding the rising sun and setting sun also appears red. Thus, at sunrise and sunrise, the sun itself as well as the surrounding sky appear red.

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. 
Answer: The sun at sunrise (early in the morning) is located near the horizon of the earth. Light from the sun near the horizon has to pass through thick layers of air and a large distance through the earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue light rays with shorter wavelength are scattered away by the particles in the atmosphere. Therefore, the light that reaches our eyes is the red light of longer wavelengths. This gives rise to the reddish appearance of the sun.
This phenomenon will not be observed by an observer on the moon because there is no atmosphere on the moon to scatter light.

Question. State the cause of dispersion of white light passing through a glass prism. How did Newton show that white light of Sun contains seven colours using two identical glass prisms. Draw a ray diagram to show the path of light when two identical glass prisms are arranged together in inverted position with respect to each other and a narrow beam of white light is allowed to fall obliquely on one of the focus of the first prism.
Answer: • White light is a mixture of lights of seven colours, red, orange, yellow, green, blue, indigo and violet.
The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. The amount of refraction depends on the speed of coloured light in glass.
• When white light consisting of seven colours falls on a glass prism, each colour in it is refracted by a different angle, with the result that seven colours are spread out to form a spectrum. The red light bends the least, while violet bends the most.
Newton’s experiment with two identical prisms:
• When a beam of white light is passed through a glass prism, a band of seven colours is formed on a white screen. This band of seven colours is called spectrum of white light.
• Newton showed that the seven coloured lights of the spectrum can be recombined to give back white light.
• First he tried to split the colours of the spectrum of white light using a prism.
• He then placed a second identical prism in an inverted position with respect to the first prism. This allowed all the colours of the spectrum to pass through the second prism. He found a beam of white light emerging from the other side of the second prism.

Question. Explain with the help of a diagram, how we are able to observe the sunrise about two minutes before the Sun gets above the horizon. 
Answer: The Sun can be seen about two minutes before sunrise because when the Sun is slightly below the horizon, the Sun’s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus due to this atmospheric refraction, the Sun appears to be raised above the horizon when actually it is slightly below the horizon.

Question. (a) What are the values of (i) near point and (ii) far point of vision of a normal adult person?
(b) A student has difficulty in reading the blackboard while sitting in the last row. What could be his defect of vision? Draw a ray diagram to illustrate this defect of vision. 
Answer: (a) (i) The nearest point up to which the eye can see an object clearly without any strain is called the near point of the eye. The near point of a normal human eye is at a distance of 25 cm from the eye.
(ii) The farthest point from the eye at which an object can be seen clearly is known as the far point of the eye. The far point of a normal human eye is at infinity.
(b) This defect of vision is called myopia. 

Question. Draw a diagram to show the dispersion of white light by a glass prism. What is the cause of this dispersion? 
Answer: • The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light.
• We get different colours of light when white light is passed through a glass prism because white light is a mixture of lights of seven colours: red, orange, yellow, green, blue, indigo and violet. The dispersion of light occurs because the angle of refraction (or angle of bending) of lights of different colours is different when passing through the glass prism.

Question. (a) What is meant by the dispersion of white light? Draw a diagram to show dispersion of white light by the glass prism.
(b) Explain why the planets do not twinkle but the stars twinkle. 
Answer: (a) Dispersion of light. The splitting of white light into its component colours on passing through a prism is called dispersion of light.
(b) The planets are much closer to the earth. A planet can be considered as a collection of large number of point-sized sources of light. So the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero thereby nullifying the twinkling effect.
On the other hand, stars twinkle because stars are point-sized sources of energy therefore the continuously changing atmosphere causes atmospheric refraction which causes variation in light.

Question. (a) What is meant by the power of accommodation of an eye?
(b) A person with a myopic eye cannot see objects beyond 1.2 m directly. What should be the type of the corrective lens used? What would be its power?
Answer: (a) The ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length or converging power of its lens is called power of accommodation. A normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focussed on the retina.
(b) Myopic eye
v = 1.2 m = –120 cm; u = ∞ (infinity); P of the corrective lens = ?
By Lens formula, 
1/v - 1/u = 1/f ⇒ 1/-120 - 1/∞ = 1/f
-1/120 - 0 = 1/f (∴ 1/∞ = 0) ⇒ -1/120 = 1/f
∴ f = –120 cm
Thus the corrective lens for the above defect is concave lens (as focal length has –ve sign) of focal length of 120 cm.
P = 1/f (in metres) = 1 x 100 /-120 = –0.83 D

Question. Why does the Sun appear reddish early in the morning?
Answer: The Sun at sunrise (early in the morning) is near the horizon. Light from the Sun near the horizon has to pass through thick layers of air and a large distance through the earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue-light of shorter wavelengths are scattered away by the particles.
Therefore, the light that reaches our eyes is of longer wavelengths, i.e., red light. This gives rise to the reddish appearance of the Sun.

Question. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
Answer: The maximum accommodation of a normal eye is reached when the object is at a distance of 25 cm from the eye. The focal length of the eye lens cannot be adjusted below this minimum limit. The rays coming from the objects which are at a distance less than 25 cm are not focussed at the retina and the image of the object appears blurred. Thus, the image cannot be seen clearly.

Question. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting
(i) distant vision, and (ii) near vision?
Answer: (i) Power, P = –5.5 D, Focal length, f = ?
P = 1/f or –5.5 = 1/f ∴ f = 1/-5.5m = – 0.18 m
Negative sign indicates that the lens is concave.
(ii) Power, P = +1.5 D Focal length, f = ?
P = 1/f or +1.5 = 1/f ∴ f =f = 1/+1.5m = +0.67 m
Positive sign indicates that the lens is convex.

Question. Why do stars twinkle?
Answer: The twinkling of a star is due to atmospheric refraction of starlight. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its 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 starlight entering the eye flickers—the star sometimes appears brighter, and at some other time, fainter, which is the twinkling effect.

Question. Due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens a certain defect of vision arises. Write the name of this defect. Name the type of lens required by such persons to improve the vision. Explain the structure and function of such a lens.
Answer: The defect of this vision is called Presbyopia. Bifocal lens is used for the correction of this defect. The upper part of this lens is concave and the lower part of the lens is Convex lens. The upper part of the lens is used to see the distant objects clearly and lower part of this lens is used to see the nearby objects clearly.

Long Answer Questions

Question. A student is unable to see clearly the words written on the blackboard placed at a distance of approximately 4 m from him.
Name the defect of vision the boy is suffering from. Explain the method of correcting this defect. Draw ray diagram for the: 
(i) defect of vision and also (ii) for its correction.
Answer: • As the student is unable to see clearly the words written on the blackboard at a distance of 4 m, the student is suffering from myopia or near-sightedness.
• This defect is corrected by using spectacles fitted with concave lens of suitable focal length is used.
The image is allowed to form at the retina by using a concave lens of suitable power.

Question. (a) Write the function of each of the following parts of human eye: cornea; iris; crystalline lens; ciliary muscles
(b) Millions of people of the developing countries of the world are suffering from corneal blindness.
These persons can be cured by replacing the defective cornea with the cornea of a donated eye. A charitable society of your city has organised a campaign in your neighbourhood in order to create awareness about this fact. If you are asked to participate in this mission how would you contribute in this noble cause?
(i) State the objective of organising such campaigns.
(ii) List two arguments which you would give to motivate the people to donate their eyes after death. 
Answer: (a) Functions of the following parts of human eye:
(i) Cornea. The front part of the eye is called cornea. It is made up of a transparent substance. The light coming from objects enters the eye through cornea.
(ii) Iris. This is a flat, coloured, ring-shaped membrane behind the cornea. Pupil is a hole in the middle of the iris. Iris controls the size of the pupil.
(iii) Crystalline lens. Eye lens is a convex lens which focuses the image of the object on the retina.
(iv) Ciliary muscles. Ciliary muscles hold the eye lens and changes the thickness of eye-lens while focussing the objects placed at different distances from the eye.
(b) (i) Objective of such campaigns. To make people aware of corneal blindness and make them realise their duties towards the society by taking pledge for eye donation.
(ii) • One pair of eyes can give eyesight to two corneal blind persons (each getting one eye), and make them see this beautiful world.
• Our eyes can live even after our death. People belonging to all age groups, even people with medical conditions like cataract, diabetes, hypertension can donate their eyes.

Question. What is atmospheric refraction? Use this phenomenon to explain the following natural events.
(a) Advanced sun-rise and delayed sun-set
Answer: Atmospheric refraction. The refraction of light caused by the earth’s atmosphere having air layers of varying optical densities is called atmospheric refraction.
(a) Advanced sun-rise and delayed sun-set. The Sun can be seen about two minutes before sunrise because when the Sun is slightly below the horizon, the Sun’s light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere. Thus due to this atmospheric refraction, the Sun appears to be raised above the horizon when actually it is slightly below the horizon.
It is also due to atmospheric refraction that we can still see the sun for about 2 minutes even after the Sun has set below the horizon. At sunset, the apparent position of the sun is visible to us and not the actual position because of the bending of light rays effect.
So, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes because of atmospheric refraction.

Question. What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the sun appears reddish at sunrise. 
Answer: Scattering of light is the phenomenon to throw light in various random directions. Light is scattered when it falls on various types of suspended particles in its path. The colour of the scattered light depends on the size of the scattering particles in the atmosphere.
— The larger particles of dust and water droplets present in the atmosphere scatter the light of longer wavelengths due to which the scattered light appears white.
— The extremely minute particles such as air molecules present in the atmosphere scatter mainly blue light present in the white sunlight.
Colour of the 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 wavelength at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky. Colour of the sun appears red at sunrise and sunset. The sun at sunrise and sunset is very near to the horizon, and near the horizon most of the blue light of shorter wave-lengths is scattered away by the particles in the atmosphere. Therefore, the light that reaches our eyes is of longer wavelengths that gives rise to the reddish appearance of the sun.

Question. Write the importance of ciliary muscles in the human eye. Name the defect of vision that arises due to gradual weakening of the ciliary muscles in old age. What type of lenses are required by the persons suffering from this defect to see the objects clearly?
Akshay, sitting in the last row in his class, could not see clearly the words written on the blackboard. When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his seat with Akshay. Salman immediately agreed to exchange his seat with Akshay. He could now see the words written on the blackboard clearly. The teacher thought it fit to send the message to Akshay’s parents advising them to get his eyesight checked. In the context of the above event, answer the following questions: 
Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?
Answer: Importance of ciliary muscles. The focal length of eye lens can be changed by changing its shape by the action of ciliary muscles. Ciliary muscles modify the curvature of the eye lens to enable the eye to focus objects at varying distances. In old age, ciliary muscles become weak and the eye lens become inflexible. As a result, the eye loses its power of accommodation. Because of this, an old person cannot see the nearby objects clearly. This leads to the defect called presbyopia. Bifocal lenses are required by the persons suffering from presbyopia in which upper part consists of a concave lens for distant vision and the lower part consists of a convex lens used to see nearby objects.
Akshay is suffering from myopia or near sightedness. Myopia is corrected by using spectacles containing concave lenses. The concave lens used for correcting myopia should be of such a focal length that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.