Human Eyes and Colourful World Class 10 Science Exam Questions

Please refer to Human Eyes and Colourful World Class 10 Science Exam Questions provided below. These questions and answers for Class 10 Science have been designed based on the past trend of questions and important topics in your class 10 Science books. You should go through all Class 10 Science Important Questions provided by our teachers which will help you to get more marks in upcoming exams.

Class 10 Science Exam Questions Human Eyes and Colourful World

Class 10 Science students should read and understand the important questions and answers provided below for Human Eyes and Colourful World which will help them to understand all important and difficult topics.

Very Short Answer

Question. Define the term power of accomodation. Write the modification in the curvature of the eye lens which enables us to see the nearby objects clearly ?
Answer : The ability of the eye lens to adjust its focal length, is called the power of accommodation. There should be a contraction of ciliary muscles, that will increase the curvature of the eye lens and becomes thicker, so the focal length of the eye lens will decrease. It will thus enable us to see the objects clearly.

Question. In the formation of spectrum of white light by a prism which colour is :
(a) deviated least ? (b) deviated most ?
Answer : (a) Red, (b) Violet.

Question. What happens to the image distance in the normal human eye when we decrease the distance of an object, say 10 m to 1 m ? Justify your answer.
Answer : The image distance will remain unaffected even if we change the object distance because the image is formed on the retina.

Question. What is persistence of vision ?
Answer : The image of any object formed on the retina persists for about 1/16 of a second. This continuance of sensation of eye for sometime even after the removal of the object is called persistence of vision.

Question. A person is advised to wear spectacles with (a) Convex lenses, (b) Concave lenses. What type of defect of vision is he suffering from ?
Answer : (a) Hypermetropia or long-sightedness.
(b) Myopia or short-sightedness.

Question. Give an example in nature which shows that sunlight may be made up of different colours.
Answer : Formation of rainbow.

Question. Where do we see (a) concave and (b) convex lens in bifocal lenses ?
Answer : (a) Concave lens—Upper part
(b) Convex lens—Lower part.

Question. Write the structure of eye lens and state the role of ciliary muscles in the human eye.
Answer : The eye lens of the human eye is a convex lens that is thick in the middle and thin from the edges. It converges the incident light rays and forms the image on retina. Role of ciliary muscles :
(a) It changes the shape of the lens in eye to help with focussing.
(b) It helps to regulate the flow of aqueous humor in eye.

Question. What do you mean by a rainbow ?
Answer : Rainbow is a natural spectrum formed in the sky after rain shower due to dispersion of light caused by suspended water droplets in atmosphere.

Question. Name the principle on which a prism form the image of an object ?
Answer : Refraction of light.

Question. What do you mean by far point of human eye ?
Answer : The most distant point at which an object can be seen clearly by the eye is called the far point of the eye.

Question. Name the type of defect of vision a person is suffering from, if he uses convex lenses in his spectacles for the correction of his vision. If the power of the lenses is +0.5 D, find the focal length of the lenses.
Answer :
The person is suffering from hypermetropia which is also knows as far-sightedness.

Human Eyes and Colourful World Class 10 Science Exam Questions

Question: State one function of the crystalline lens in the human eye.
Answer: The crystalline lens of human eye focuses the light that enters the eye and form the image on the retina.

Question: State one function of iris in human eye.
Answer: Iris is a dark muscular diaphragm that controls the size of the pupil.

Question. What is scattering of light ? Why is the colour of the clear sky blue ? Explain.
Answer : (a) The phenomenon in which a part of the light incident on a particle is redirected in different directions.
(b) When sunlight passes through the atmosphere, its fine particles scatter the blue colour more strongly than red. So, the scattered blue light enters our eyes.
Hence the sky appears blue.

Question: What will be the colour of the sky when it is observed from a place in the absence of any atmosphere?
Answer: If the earth had no atmosphere, there would not have been any scattering. Then, the sky would look dark.

Question: Name the part of our eyes that helps us to focus near and distant objects in quicksuccession.
Answer: Crystalline lens

Question: Why is the colour of clear sky blue?
Answer: The colour of clear sky is blue due to the scattering of light having shorter wavelength by the air molecules present in the atmosphere

Question: Give an example of a phenomenon where Tyndall effect can be observed.
Answer: The phenomenon of scattering of light by the colloidal particle give rise to Tyndall effect.

Short Answer

Question. (a) A person is suffering from both myopia and hypermetropia.
(i) What kind of lenses can correct this defect?
(ii) How are these lenses prepared ?
(b) A person needs a lens of power + 3D for correcting his near vision and – 3D for correcting his distant vision. Calculate the focal lengths of the lenses required to correct these defects.
Answer : (a) (i) If a person suffer from both myopia and hypermetropia, such people often require bifocal lenses.
(ii) A common type of bifocal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens.
It facilitates near vision.
(b) For correcting distant vision;
Power, P = – 3 D
Using P = (1/f) where f = focal length of the lens.

The focal length of the lens for correcting distant vision = – 0.33 m.
Minus sign of focal length tells us that it is a concave lens.
For correcting near vision
Power, P = + 3 D
Using P = (1/f) where f = focal length of the lens.
⇒ f = (1/3) = + 0.33 m
The focal length of the lens for correcting distant vision = + 0.33 m.
Plus sign of focal length tells us that it is a convex lens.

Question. What is atmospheric refraction ? List two phenomena which can be explained on the basis of atmospheric refraction.
Answer : In atmosphere, there are layers of different densities and refractive indices, when light ray is passed through these layers refraction of light takes place which is
called atmospheric refraction.
Two phenomenon that can be explained on the basis of atmospheric refraction are :
(a) Twinkling of stars.
(b) Early sunset and delayed sunrise.

Question. What happens to a beam of white light when it gets refracted through glass prism ? Which colour deviates the most and the least after refraction through a prism ?
What is likely to happen if a second identical prism is placed in an inverted position with respect to the first prism. Justify your answer.
Answer : When white light is refracted through a glass prism, it gets split into its constituting colours at different angles.
This phenomenon is called dispersion of light. Forming a rainbow,

Least deviated colour is red whereas most deviated colour is violet.
When second identical prism is placed in an inverted position with respect to first prism, recombination of the spectrum will take place and white light will be obtained.

Question. (a) With the help of labelled ray diagram show the path followed by a narrow beam of monochromatic light when it passes through a glass prism.
(b) What would happen if this beam is replaced by a narrow beam of white light ?
Answer :

PE – Incident ray
EF – Refracted ray
FS – Emergent ray
∠A – Angle of the prism
∠i – Angle of incidence
∠r – Angle of refraction
∠e – Angle of emergence
∠D – Angle of deviation
(b) The prism will split the incident white light into a band of colours. The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Question. What is a rainbow ? Draw a labelled diagram to show the formation of a rainbow.
Answer : The rainbow is a natural phenomenon in which the white light or sunlight splits into beautiful 7 colours by the water droplets which remain suspended in air after rain.

Question. (a) What is myopia ?
(b) How can it be corrected ?
Answer : (a) The defect of human eye in which a person can see the objects lying at short distances clearly but cannot see the far objects distinctly is called myopia or short-sightedness.
(b) A myopic eye can be corrected by using spectacles with concave lens of suitable power or focal length.

Question. State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms show that white light is made of seven colours ? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical prisms arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.
OR
What is “dispersion of white light” ? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why is it essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other ?
Answer : When a beam of white light enters a prism, it gets refracted at point and splits into its seven constituent colours. This splitting of the white light occurs because of different angles of bending for each colour and this different angles of bending occurs because different components of light have different refractive indices when passing through the glass prism. Firstly, Newton made white light to fall on a prism, that caused dispersion of white light into seven colours.
Newton then placed an inverted prism in the path of a colour band of seven colours. Only a beam of white light came out from the second prism. So, Newton concluded that white light comprises of seven component colours.

The phenomenon of splitting of white light into its constituent colours on passing through a prism is known as the dispersion of white light. This splitting of the light rays occurs because of the different angles of bending for each colour and this different angles of bending occurs because different component of light faces different refractive index (or speed of different colours is different) when passing through the glass prism.

It is essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other so that the second prism completely nullifies the dispersion caused by the first prism and we get pure white light.

Question. A student needs spectacles of power – 0·5 D for the correction of his vision.
(a) Name the defect in vision the student is suffering from.
(b) Find the nature and focal length of the corrective lens.
(c) List two causes of this defect.
Answer : (a) Myopia.
(b) Concave lens with the focal length of 200 cm
Given, P = – 0·5 D
We have,

Question. (a) A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain
(i) Why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.
Answer : (a) Two possible reasons due to which this defect of vision may have arisen are :
1. Increase in curvature of the lens.
2. Elongation of the eyeball.
(i) A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of the retina as shown below :

In the given case, student’s far point is 5 m. So, image of the object placed beyond 5 m from his eyes is formed in front of the retina and object appears blurred. That is why the student is unable to see distinctly the objects placed beyond 5 m from his eye.
(ii) Since a concave lens has an ability to diverge the incoming rays. Therefore, it is used to correct this defect of vision. The image is formed at the retina by the use of a concave lens of suitable power as shown.

Question. Explain why the planets do not twinkle.
Answer : The planets are much closer to the earth and thus seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, 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. Hence, the planets do not twinkle.

9. What is ‘dispersion of white light’ ? State its cause.
Draw a ray diagram to show the dispersion of white light by a glass prism.
Answer : The phenomenon of splitting of white light into its seven constituent colours, when it passes through a glass prism, is called the dispersion of light.

When a beam of white light enters a prism, it gets refracted at point and splits into its seven constituent colours i.e., violet, indigo, blue, green, yellow, orange and red i.e., VIBGYOR. This splitting of light rays occurs because of the different angles of bending for each colour and this different angles of bending occurs because different components of light have different refractive indices or speed of different colours is different in glass when passing through the glass prism. When a beam of sunlight is allowed to fall on one of the rectangular surfaces of the glass prism, we obtain a coloured spectrum with red and violet colour at its extremes.

Question. (a) What is hypermetropia ?
(b) How can it be corrected ?
Answer : (a) The defect of the human eye in which a person can see the objects lying at long distances clearly but cannot see the nearby objects distinctly is called hypermetropia or long-sightedness.
(b) To correct a hypermetropic eye, the person is allowed to wear spectacles with a convex lens of suitable power or focal length.

Question. Name the four parts labelled as 1, 2, 3 and 4 in the given diagram.

Answer :
(1) Retina, (2) Ciliary muscles, (3) Pupil, (4) Crystalline lens.

Question. Write about power of accommodation of human eye. Explain why the image distance in the eye does not change when we change the distance of an object from the eye ?
Answer : The ability of the eye to see object, at varying distances, clearly by adjusting the focal length of its lens is called power of accommodation.
The focal length of the human eye can change i.e. increase or decrease, depending on the distance of objects and due to this the image distance in the eye does not change. When the distance of an object is changed from the eye, it is the ciliary muscles that modify the curvature of the lens to change its focal length.

Question. When we increase the distance of an object from the eye, what happens to the image distance in the eye ?
Answer : For a normal eye, image distance in the eye is fixed, being equal to distance of retina from the eye lens.
When we increase the distance of an object from the eye, the focal length of eye lens is changed on account of accommodation power of the eye, so as to keep the image distance constant.

Question. State the main cause of atmospheric refraction of light.
With the help of a diagram, explain the phenomenon of twinkling of stars.
Answer : Refraction of light caused by the earth‘s atmosphere due to change in the refractive indices of different layers at atmosphere. Twinkling of stars : The twinkling of a star is due to atmospheric refraction of starlight. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. Further, this apparent position of the star is not stationary, but keeps on changing slightly.

Question. (a) Draw a figure which show the arrangement for observing the phenomenon of scattering of light in the laboratory.
(b) What colours would you observe in the experiment ? Why ?
Answer : (a) An arrangement for observing the scattering of light in the laboratory is as shown below.

(b) (i) On the screen, first orange red colour and then bright crimson red colour patch is observed.
(ii) From the other three sides of colloidal solution of sulphur in a glass tank (T), blue colour is observed.
This is because the very fine colloidal sulphur particles scatter away the blue colour from the path of beam and only red colour (least scattered) of the beam of white light reaches the screen through the solution.

Question. If Ram, a myopic person uses spectacles of power – 0.5 D, then what will be the distance of far point of his eye ?
Answer : Power, P = – 0.5 D
Let x be the distance of the far point.
The focal length, f = – x
Power of a lens is given by,

Question. Why does the sun appear white at noon and reddish at the sunrise/sunset ? Explain.
Answer : Light from the sun overhead would travel relatively shorter distance. At noon, sun appears white as only little of the blue and violet colours are scattered. However, near the horizon most of the blue light and shorter wavelength are scattered away by the particles.
Therefore, the light that reaches our eyes is of longer wavelength which gives rise to the reddish appearance of sun.

Question: Draw a ray diagram to show the refraction of light through a glass prism. Mark on it (a) the incident ray, (b) the emergent ray and (c) the
angle of deviation.
Answer:

i = angle of incidence
(a) PE = incident ray
(b) FS = emergent ray
(c) ∠D = angle of deviation

Question: When we place a glass prism in the path of a narrow beam of white light a spectrum is obtained. What happens when a second identical prism is placed in an inverted position with respect to the first prism? Draw a labelled diagram to illustrate it.
Answer: Spectrum : The band of the colored components of a light beam is called its spectrum. We recombine the components of white light by placing a second identical prism in an inverted position with respect to first prism.

On passing white light to the first prism it splits into colours of the spectrum. Then this spectrum is allowed to pass through a second identical prism in an inverted position with respect to the first prism. Then, we found a beam of white light.

Question: A narrow PQ of white light is passing through a glass prism ABC as shown in the diagram. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE. P

(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 the splitting up of the white light into its constituents colours is called dispersion of light. Dispersion of light is caused due to, different constituents colours of light after different refractive indices to the material of the prism.
(ii) The formation of rainbow is caused by the dispersion of the white sunlight into its constituent colours.
(iii) Based on the dispersion of white light into its constituents colours, we can conclude that (a) The white light consists of seven colours
(b) The violet light suffers maximum deviations and the red light suffers minimum deviation.

Question: Draw a ray diagram to show the formation of a rainbow and mark the point where (i) dispersion, (ii) internal reflection occurs.
Answer: (i) Point A denotes dispersion and (ii) point B denotes internal reflection.

Question: Explain the formation of rainbow in the sky with the help of a diagram.
Answer: After a rain-shower, the sunlight gets dispersed by tiny droplets, present in the atmosphere. The water droplets acts like a small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to dispersion of light and internal reffection different colours reach the observer’s eye.

Question: Give reasons:
(i) The extent of deviation of a ray of light on passing through a prism depends on the colour.
(ii) Lights of rered colour are used for danger signals.
Answer: (i) The extent of deviation of a ray of light on passing through a prism depends on the colour because the refractive index of glass for different colour is different. It depends on wavelength of a particular light.
(ii) Since the wavelength of light is maximum in the spectrum, its penetration power in the air is maximum and so we can see red colour from further distances. Thus, danger signal uses red colour.

Question: Draw a labelled ray diagram to illustrate the dispersion of a narrow beam of white light when it passes through a glass prism.
Answer: Splitting of white light into its seven constituent colours due to refraction is known as dispersion of white light.
Cause of dispersion : When a beam of white light enters a prism, it gets refracted and splits into seven constituent colours. The splitting of the light ray occurs due to the different bending angle for each colour. Thus, each colour ray when passing through the prism bends at different angles with respect to the incident beam, thus gives rise to a
spectrum.

Question: What is meant by the dispersion of white light? Draw a diagram to show dispersion of white light by the glass prism?
Answer:

(i) The phenomenon of the splitting up of the white light into its constituents colours is called dispersion of light. Dispersion of light is caused due to, different constituents colours of light after different refractive indices to the material of the prism.
(ii) The formation of rainbow is caused by the dispersion of the white sunlight into its constituent colours.
(iii) Based on the dispersion of white light into its constituents colours, we can conclude that
(a) The white light consists of seven colours
(b) The violet light suffers maximum deviations and the red light suffers minimum deviation

Question: Explain in brief the reason for each of the following:
(a) Advanced sun-rise
(b) Delayed sun-set
(c) Twinkling of stars
Answer: (a, b) : The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes afer the actual sunset because of atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun. Figure shows the actual and apparent positions of the Sun with respect to the horizon. The time difference between actual sunset and the apparent sunset is about 2 minutes. The apparent flattening of the Sun’s disc at sunrise and sunset is also due to the same phenomenon

(c) Since the stars are very distant, they are approximately 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: Explain giving reason why the sky appears blue to an observer from the surface of the Earth. What should the appearance of the sky be during the day for an astronaut staying in the international space station orbiting the Earth? State reason to justify your answer.
Answer: When sunlight passes the atmosphere, the fine particles in air scatter blue colour more strongly than red. This scattered blue light enters our eye and the colour of clear sky appears blue. For an astronaut staying in the international space station orbiting the Earth, the appearance of the sky will be black due to absence of air molecules to scatter the light coming from the Sun.

Question: A star appears slightly higher (above) than its actual position in the sky. Illustrate it with the help of a labelled diagram.
Answer: A star appears slightly above than its actual position in the sky. Since the starlight, on entering the earth’s atmosphere undergoes refraction continuously in a medium of gradually changing refractive index, before it reaches the earth. Since the atmosphere bends starlight towards the normal, the star appears slightly above than its actual position.

Question: With the help of a labelled diagram,explain why the sun appears reddish at the sun-rise and the sun-set.
Answer:

At sun-rise and the sun-set, light from the sun passes through thicker layers of air and larger distance in the earth’s atmosphere. As the red colour has longest wavelength hence, it is least scattered by the air and dust particles. So, the sun appears reddish.

Question: State the difference in colours of the sun observed during sunrise/sunset and noon.Give explanation for each.
Answer: During sunrise or sunset, the suns rays pass through a maximum length of the atmosphere. Most of the blue and shorter wavelength get scattered. Only the red colour of light reaches the observer.That is why the Sun observed during sunset and sunrise appear red. At noon, the distance to be travelled is least. All wavelengths are scattered equally and hence sun appears white.

Long Answer:

Question. When do we consider a person to be myopic or hypermetropic ? List two causes of hypermetropia.
Explain using ray diagrams how the defect associated with hypermetropic eye can be corrected.
Answer : Myopia is the defect in vision in which a person cannot see the distant objects clearly whereas in hypermetropia is the defect in which a person cannot see nearby objects clearly. Hypermetropia is caused due to :
(a) Decrease in converging power of eye-lens.
(b) Too short eye ball.
In a hypermetropic eye, the image of near by object lying at normal near point N (at 25 cm) is formed behind the retina.

Hypermetropic eye can be corrected using convex lenses. When a convex lens of suitable power is placed in front of hypermetropic eye, then the diverging rays of light from the object are converged first by the convex lens used. This form a virtual image of the object at another near point N′.
Now, the rays can be easily focused by the eye lens to form an image on retina.

Question. (a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
(b) Why do stars twinkle ? Explain.
Answer : (a) The boy is suffering from myopia.
This defect is caused :
(i) due to increase in length of eyeball, and
(ii) decrease in focal length of eye lens, when the eye is fully relaxed.
Correction : The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure (a).

As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the myopic eye. The far point of such an eye is near to the eye as shown in figure (b).

This defect can be corrected by using a concave lens (minus powered) of suitable focal length. So, a man suffering from this defect wears spectacles having concave lens of suitable focal length. The concave lens diverges the rays of light entering the eye from infinity. Hence, this lens makes the rays of light appear to have come from the far point (O’) of the defective eye as shown in figure (c).

(b) 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 approximate pointsized sources of light. As the path of light rays coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers i.e., the star sometimes appear brighter and at some other time, fainter, which is the twinkling effect.

Question. (a) What is scattering of light ? Explain how the colour of the scattered light depends on the size of the scattering particles.
(b) Explain the reddish appearance of the Sun at sunrise or sunset. Why does it not appear red at noon ?
Answer : (a) Scattering of light is the phenomenon in which a part of the incident light is dispersed in different directions.
Dependence of colour and scattered light on the size of particles :
(i) When the particles like dust and water droplets present in the atmosphere are large in size, the scattered light appears white.
(ii) When the particles are extremely minute in size, they will scatter blue light present in the white sunlight.
(b) The reddish appearance of the sun at sunrise and sunset is due to the scattering of blue colour present in the sunlight away from our line of sight and leaves behind mainly red colour of the direct sunlight which reaches human eye.
The reason for Sun not appearing red at the noon is that the light has to travel a relatively shorter distance through the atmosphere to reach us and therefore, only a litte of blue colour of the white light is scattered.

Question. (a) Draw a ray diagram to explain the term angle of deviation.
(b) Why do the component colours of incident white light split into a spectrum while passing through a glass prism, explain.
(c) Draw a labelled ray diagram to show the formation of a rainbow.
Answer : (a) The angle between incident ray, produced forward and emergent ray, produced backward is called angle of deviation D.

(b) The splitting up of white light into its constituent colours on passing through a refracting medium like a glass prism is called dispersion of light. The dispersion of white light occurs because different colours of light bend through different angles with respect to the incident ray, as they travel with different speeds in a prism. The red light bends the least while the violet light the most as shown below :

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. State the possible causes of this defect and explain the method of correcting it.
Answer : The boy is suffering from myopia.
This defect is caused :
(a) due to increase in length of eyeball, and
(b) decrease in focal length of eye lens, when the eye is fully relaxed. Correction : The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure (a).

As the image of the object lying a infinity is not formed on the retina of the eye, so such object cannot be seen clearly by the myopia eye. The far point of such an eye is near to the eye as shown in figure (b).

This defect can be corrected by using a concave lens of suitable focal length. So, a man suffering from this defect wears spectacles having concave lens of suitable focal length The concave lens diverges the rays of light entering the eye from infinity. Hence, this lens makes the rays of light appear to come from the far point (O’) of the defective eye as shown in figure (c).

Question. Does myopia or hypermetropia imply necessarily that the eye has partially lost its ability of accommodation ? If not, what cause these defects of vision ?
Answer : No, a person may have normal ability of accommodation and yet he may be myopic or hypermetropic. In fact,
myopia may arise when length of eye ball is elongated and hypermetropia may arise when length of eye ball gets shortened.

Question. (a) What is presbyopia ? State its cause. How is it corrected ?
(b) Why does the Sun appear reddish early in the morning ? Explain with the help of a labelled diagram.
Answer : (a) Presbyopia : Defect of vision when a person is unable to see nearby as well as far off objects clearly.
The causes of presbyopia are :
(a) Gradual weakening of the ciliary muscles.
(c) Diminishing flexibility of the eye lens. It can be corrected by using bifocal lens.
(b) This is because, at the time of sunrise, the sun is near the horizon and so, the sunlight has to travel a large distance through the atmosphere to reach us.
During this travel, most of the shorter wavelengths light present in it are scattered out from our line of sight and only red light having larger wavelength is able to reach our eye due to less scattering. Hence, the sun appear reddish early in the morning.

Question. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 0.8 m. What is the power of the lens required to correct this defect ? Assume that the near point of the normal eye is 25 cm.
Answer : Hypermetropia can be corrected by using a convex lens of appropriate power. The ray diagram is as follows :

Here,
The object distance, u = – 25 cm = – 0.25 m
The image distance, v = – 0·8 m

Question. An old person finds it difficult to see nearby objects comfortably and distinctly without corrective eye glasses.
(a) What defect of vision is he suffering from ? What is it ?
(b) List two causes for the development of this defect.
(c) What kind of lens will be required to see clearly the nearby as well as distant objects ? Give reasons.
(d) How is the eye defect of old person differing from near-sightedness and far-sightedness ?
Answer : (a) Presbyopia
(i) Presbyopia is a condition that occurs as part of normal ageing.
Due to loss of power of accommodation of he eye, with age, objects at a normal near working distance will appear blurry. The near point gradually recedes away. This defect of eye is called presbyopia.
(ii) Sometimes, a person may suffer from both myopia and hypermetropia.
(b) Presbyopia is caused due to :
(i) Weakening of ciliary muscles, and
(ii) Eye lens becomes less flexible and elastic,
i.e., reducing ability of eye lens to change its curvature with the help of ciliary muscles.
(c) Bifocal lens will be required to see clearly nearby as well as the distant object. For myopic defect, upper part of bifocal lens consists of a concave lens used for distant vision and to correct hypermetropia, lower part of bifocal lens consists of a convex lens.
It facilitates near vision.
(d) Presbyopia occurs when the natural lens in the eye loses its flexibility with ageing while farsightedness and near-sightedness are related to
the change in shape of the eyeball and are caused by the genetic and the environmental factors.

Question. (a) Explain the following terms used in relation to defects in vision and correction provided by them :
(i) Myopia, (ii) Bifocal lenses, (iii) Far-sightedness.
(b) Why is the normal, eye unable to focus on an object placed within 10 cm from eye ?
Answer : (a) (i) Myopia or short-sightedness is caused due to excessive curvature of the eye lens or elongation of the eyeball. The image forms in front of the retina. So, a concave lens is used to correct it.
(ii) Bifocal lens is used to correct presbyopic eye. The upper portion of a bifocal lens is concave and its lower portion is convex. The concave lens is used for viewing long distant objects and the convex lens is used for reading purposes. This problem arises with age.
(iii) Far-sightedness or hypermetropia is caused due to greater focal length of eye lens or eye ball becomes smaller. The image forms beyond the retina and can be corrected by using a convex lens.
(b) The least distance of distinct vision for normal eye is 25 cm. The focal length of the eye lens cannot by further decreased by ciliary muscles to form an image on the retina for the objects nearer than 25 cm. Therefore, the normal eye is unable to focus on an object placed within 10 cm from the eye.

Question. What is spectrum of white light ? Why do different coloured rays deviate at different angles on passing through a glass prism ? Explain the method of recombining the different components of white light.
Answer : (a) The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is called spectrum of white light.
(b) The rays of different colours have different speed in a certain medium. Therefore, the refractive index of prism is different for light of different index of prism is different for light of different colours and thus the white light gets dispersed into differed colours.
(c) When second identical prism is placed in an inverted position with respect to first prism, recombination of the spectrum will take place and white light will be obtained.

Question. (a) Write the functions of each of the following parts of the human eye :
(i) Cornea (ii) Iris
(iii) Crystalline (Eye) lens
(iv) Ciliary muscles (v) Retina
(b) A person is unable to see distinctly the objects closer than 1 m. Name the defect of vision he is suffering from. Draw ray diagrams to illustrate the cause of the defect and its correction by suitable lens.
Answer : Functions of the given parts of the human eye :
(a) (i) Cornea- To refract the light rays falling on the eye
(ii) Iris- To control the amount of light entering the eye.
(iii) Crystalline lens- To focus the incoming rays on the retina.
(iv) Retina- To act as screen and send signal to the brain via optic nerve
(b) Person is suffering from hypermetropia or longsighted ness.

Question. Name the phenomenon responsible for the early sunrise and delayed sunset. Explain with the help of a diagram the reason why the sun is visible to us about 2 minutes before the actual sun-rise and about 2 minutes after the actual sunset.
Answer : Atmospheric refraction is responsible for the early sunrise and delayed sunset.
(a) Early Sunrise : When the sun is slightly below the horizon, light rays coming from the sun travel from the rarer to denser layer of air. Because of atmospheric refraction of light, light appears to come from a higher position above the horizon.
Thus, sun appears earlier than actual sunrise.
(b) Delayed sunset : Same reason, as similar refraction occurs at the sunset.

Question: Do you know that the corneal-impairment can be cured by replacing the defective cornea with the cornea of the donated eye? How and why should we organise groups to motivate the community members to donate their eyes after death?
Answer: Yes, we know that the corneal – impairment can be cured by replacing the objective cornea with the cornea of the donated eye. We can provide the importance of eye donation to the community members. Our eyes can live even after our death.
By donating our eyes after die, we can light the life of a blind person. The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and colours around us It is however, impossible to identify colours while closing the eyes. Thus of all the sense organs, the human eye is the most significant one as it enables as to see the beautiful colourful word around us. Hence, we should donate our eyes after death.

Question: (a) Why do the component colours of incident white light split into a spectrum while passing through a glass prism, explain.
Answer: (a) When a beam of light incidents on a prism, it first gets refracted and splits into seven constituent colors. The splitting of the light ray occurs due to the different bending angle for each colour. Thus each colour ray when passing through the prism bends at different angles with respect to the incident beam. This gives rise to the formation of the spectrum.

Question: A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why it isso.
Answer: A glass prism is able to produce a spectrum but glass slab doesn’t produce spectrum because in case of a prism, when a white light passes through each of the constituent wavelength of light undergoes different extent of deviation. This result in dispersion of white light. On the other hand, when the light enters through the parallel sides of glass slab, each constituent wavelength of white light doesn’t undergo any net deviation. Hence, the white light does not split into constituent spectrum.

Question: Draw a ray diagram to explain the term angle of deviation.
Answer: The emergent ray bends at an angle to the direction of the incident, thus the angle between them is known as angle of deviation (D)

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