Class 10 Science Chapter 10 Board Questions

The retina behaves like a light sensitive screen, on which real and inverted image of any object situated in front of eye is formed.

Retina

(i) Pupil.
(ii) Retina.

It provides the fine adjustment of focal length of eye lens system so as to focus images of objects situated at different distances on the retina.

Iris control the size of pupil.

The pupil is a small circular transparent aperture hose size is controlled by iris.

It regulates and controls the amount of light entering the eye.

In bright sunlight he sizes of the pupil is small. When one enters a less lighted room, it takes time for the pupil to expand in size to adjust for dim light. Hence the person feels difficulty in seeing for some time but later on he is able to see objects.

Accommodation or the accommodating power.

To adjust/modify the shape (curvature) of eye lens so that its focal length can be increased or decreased.

The minimum distance at which objects can be seen distinctly without much strain on the eye.

Image distance remains unchanged.

When one shifts looking from a nearby object to a distant object the focal length of the eye lens increases.

(i) 25cm
(ii) infinity

It is the property of eye lens to adjust its focal length so as to focus various objects situated at different distances in front of the eye at the retina.

(a) Cornea
(b) Ciliary muscles.

Nearsightedness

Concave lens.

(i) Excessive curvature of the eye lens.
(ii) Elongation of the eyeball.

The boy is suffering from myopic.

Hypermetropia is the defect of vision in which of vision in which a person can see distant objects distinctly but cannot see nearby objects clearly.

(i) The focal length of eye length is too long.
(ii) Eyeball has become too small.

Hypermetropia

The person is suffering from myopia.

The old person is suffering from presbyopia.

Priya is suffering from myopia.

The defect of vision is presbyopia.

Cataract.

Because light of different colour bend through different angles while passing through a prism.

(i) Red light is deviated the least and
(ii) Violet light is deviated the most.

Dispersion and reflection of light.

On wavelength of light.

Atmospheric refraction.

Tyndall effect is seen when a fine beam of sunlight enters a smoke filled room through a small hole.

Scattering of light depends on the size of scattering particles and the wavelength of the light.

Black

Scattered sunlight appears white.

Because there is no scattering of light due to lack of atmosphere.

Red light is least scattered by fog or smoke and can be easily seen for a distance.

Cornea is the outermost bulged out thin transparent layer of eye and provide most of the refraction of the light entering into the eye.
The iris controls the size of the pupil of eye.
The crystalline lens provides the finer adjustment of focal length required so as to focus objects situated at difference distances in front of the eye on the retina.
The ciliary muscles help in controlling the curvature of crystalline lens and thus can change the power of the crystalline lens.

(a) The iris controls the size of the pupil. It adjusts in size and therefore helps in regulating the amount of light entering the eye through a variable aperture pupil. When the light is very bright the pupil becomes very small. However, in dim light it opens up completely through the relaxation of the iris.
(b) The crystalline eye lens consists of a fibrous jelly like material. It curvature can be modified to some to some extent by the ciliary muscles. The change in the curvature of eye lens can changed its focal length. When the muscles are relaxed the lens is thin and its focal length is more. When the ciliary muscles contract and the eye lens become thicker. Consequently, the focal length of eye lens decreases.

Accommodation power is the property of the eye lens to adjust its focal length so as to focus objects situated at different distance from the eye on the retina. When the ciliary muscles are relaxed the eye lens become thin and its focal length is maximum and equal to the diameter of the eye ball. In this condition one can see distant object clearly.
At the time of looking at nearby objects the ciliary muscles of eye contract and eye lens becomes thicker. Consequently, focal length of eye lens decreases and nearby objects are clearly focused at the retina. There are definite limits of accommodation power for a healthy normal eye. The farthest distance up to which eye can see object clearly is called the far point of eye and its value is infinity. The minimum distances up to which an eye can see distinctly, is known as near point of eye and its value is 25 cm for a normal eye.

A person is said to have a myopic vision if he can see objects situated near the eye clearly but cannot see distant objects. If a person can see clearly upto a distance x from the eye, then it means that from the eye then it means that far point of eye has shifted from infinity to a point O situated at distance x from the eye. Naturally light rays coming from a distance objects are focused in front of retina of eye.
Two possible causes of myopia are:
(i) Either the power of the eye lens has become more than its normal value due to excessive curvature of the cornea.
(ii) Elongation of eyeball due to some genetic defect.
To rectify this defect a concave lens of focal length f is used which may from the virtual image of the distant object at far point of defective eye so that now the defective eye may form the image at the retina.

Presbyopia is a defect of vision generally occurring in person of advance age. With ageing the ciliary muscles are gradually weakened and the flexibility of the crystalline lens diminishes. Consequently, the power of accommodation of eye decreases and the person cannot read comfortably and distinctly.
Presbyopia can be rectified by the use of convex lens of appropriate power. Sometimes due to weakening of ciliary muscles the old person develops symptoms of myopia as well as hypermetropia. In such cases bi-focal lenses are to be used.

Both are condition of long sightedness and are on account of increase focal length of the eye lens. In hypermetropia the eye lens becomes thin at thin at the center or the eyeball becomes shorter. On the other hand, if the defect arises due to weakness of ciliary muscles so that they are unable to reduce the focal length the defect is called presbyopia which is usually present in old age.
Thus problem and rectification of both the defects of vision is same but their causes are different

Hypermetropia or long-sightedness is that defect of vision in which the defective eye can see distant objects distinctly but is unable to see distinctly an object placed near his eye. For a nearby object the name is formed behind the retina.
Two possible causes of this defect of vision are:
(i) The power of eye lens is less due to less curvature of cornea.
(ii) The size of eyeball is shortened.
The hypermetropia defect can be corrected by using a converging lens of appropriate power.

Two main defect of vision their cause and correction are as follows:
(1) Myopia – In myopia or near-sightedness a person can see nearby object clearly but cannot see distant objects distinctly. For a myopic eye the far point is not at infinity but shifted nearer to the eye.
Myopia arises due to either(i) excessive curvature of the cornea or (ii) elongation of the eyeball. Myopia can be corrected by using a concave lens whose focal length has same numerical value as the distance of far point the defective eye.
(2) Hypermetropia – In hypermetropia or long-sightedness, a person a person can see distant object distinctly but cannot see nearby objects so clearly. For a long-sightedness, eye the near point is not 25 cm but has shifted away from the eye.
Hypermetropia arises either because (i) the focal length of eye lens too long larger or (ii) contraction of the eyeball.
Hymetrometropia can be corrected by using suitable convex lens which forms virtual image of the object situated at 25 cm at the near point of defective eye so that now the eye lens can focus it on the retina.

When a beam of white light passes through a glass prism it splits up into its constituent seven colours. The splitting of white light into its constituent colours when light passes through a dispersive medium is called dispersion of light. The seven colours, usually expressed VIBGYOR constituent the spectrum of white light.
Cause of Dispersion: When a beam of white light enters a glass prism, the light ray bends towards the normal on entering into glass. However different colours of light bend through different angles with respect to the incident ray. The red lights bend the least while the violet light bends the most, So rays of different colours emerge along different paths and thus become distinct. Hence dispersion is caused and spectrum is formed.

Stars are very far away from the earth and behaves as almost a point object. The atmosphere is made of several layers and their refraction indices keep on changing continuously. So the light rays coming from the star keep on changing their paths continuously. As a consequence, the number of rays with time and the stars appear twinkling.
A planet is comparatively nearer to the earth and subtends a larger angle at the eye. So it may be considered as a collection of larger number of point sized objects. Due to atmospheric refraction, quantity of light coming from any one-point sized object change with time but the total light entering the observer eye due to all these point objects remain almost the same. As a result, the planet does not twinkle.

Atmospheric air layer just near the earth surface is comparatively denser and upper layer of atmosphere are successively rarer and rarer. Hence a light ray passing through atmospheric air undergoes refraction. Since the physical condition of air are not stationary the apparent position of the distant object, as seen through the air fluctuates. It is known as an effect of atmospheric refraction. Light coming from a distant star on entering into the earth atmosphere gradually bends towards the normal on account of atmospheric refraction. Hence the star appears slightly higher than its actual position when viewed near the horizon.

As we go up and up in earth atmosphere it goes on becoming rarer and rarer. As a result, the atmospheric layer near the earth surface has maximum refractive index and the refractive index gradually decreases with increasing in height. When light ray from a star enters into earth atmosphere it travels from rarer to denser medium and hence continuous to bend towards the normal. As a result, an observer on earth considered the apparent position of star to be at a higher altitude.

The air becomes rarer as its height above the earth increases. Its refractive index decreases. A ray of light from the sun when it enters the atmosphere at the horizon gets refracted from rare to a denser medium. The rays therefore gradually bend toward the normal and the sun appear to be raised. As a result, the sun is visible to an observer nearly two minutes before actual sunrises at the horizon. Similarly, even after actual sunset, Sun is seen by us for about 2 minutes. Thus in effect Sun is seen for 4 minutes more. It means that aperture duration of day has increased by 4 minutes than its actual duration.

When a beam of white light passes through a glass prism it splits up into its constituent seven colours. The splitting of white light into its constituent colours when light passes through a dispersive medium is called dispersion of light. The seven colours, usually expressed VIBGYOR constituent the spectrum of white light.

Scattering of light is the phenomenon of interplay of light with minutes sized particles. As a result of such interplay light spreads in different directions and also becomes visible. The earth atmosphere is a heterogeneous mixture of minute particles including smoke tiny water droplets suspended dust particles and molecules of air. When a beam of light strikes such fine particles the path of beam becomes visible due to scattering.
The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light but particles of larger size scatter light of longer wavelength also. For large enough size of scattering particles, the scattered light may appear white.

Scattering of light is the phenomenon of interplay of light with minutes sized particles. As a result of such interplay light spreads in different directions and also becomes visible. The earth atmosphere is a heterogeneous mixture of minute particles including smoke tiny water droplets suspended dust particles and molecules of air. When a beam of light strikes such fine particles the path of beam becomes visible due to scattering.
(i) At the time of sunrises the sun is situated near the horizon. Light from the sun therefore covers a larger distance in the earth atmosphere before reaching an observer eyes. During it passes through atmosphere blue light is mostly scattered away and so the sun appears reddish.
(ii) During day time light from sun is scattered from air molecules in all directions. The amount of scattering is more for blue light because its wavelength is less. Hence the clear sky appears blue.

The air molecules and other fine particles present in earth atmosphere have size even smaller than the wavelength of visible light. As a result, these are more effective in scattering light of shorter wavelength at the blue end of spectrum. Amount of scattering of red light is comparatively very small. So when sunlight passes through atmosphere off earth, blue light is scattered much more and spread in all directions. As a result, the sky appears as blue to us. If there had no atmosphere on the earth, then there would have been no scattering of sunlight and the sky would have looked dark.

(a) The function of given parts is stated below:
(i) Cornea is the outer bulged out thin transparent layer of eye and provide most of the refraction for the light entering into the eye.
(ii) The iris controls the size of the pupil of eye.
(iii) The crystalline lens provides the finer adjustments of focal length required so as to focus objects situated at different distances in front of the eye on the retina.
(iv) The ciliary muscles help in controlling the curvature of crystalline lens and thus can change the power of the crystalline lens.
(b) In the early morning the Sun is situated near the horizon. Light from the sun passes through thicker layers of air and covers a larger distance in the earth’s atmosphere before reaching our eye. While passing through atmosphere blue light is mostly scattered away and the sun appear reddish brown.
The phenomenon is not observed by an observer on the moon because the Moon has no atmosphere of its own and hence no scattering of light is possible.

(a) The boy is suffering from myopia (near sightedness). When the ciliary muscles are relaxed the eye lens become thin and its focal length is maximum and equal to the diameter of the eye ball. In this condition one can see distant object clearly.
At the time of looking at nearby objects the ciliary muscles of eye contract and eye lens becomes thicker. Consequently, focal length of eye lens decreases and nearby objects are clearly focused at the retina. There are definite limits of accommodation power for a healthy normal eye. The farthest distance up to which eye can see object clearly is called the far point of eye and its value is infinity. The minimum distances up to which an eye can see distinctly, is known as near point of eye and its value is 25 cm for a normal eye.
(b) Stars twinkly due to atmospheric refraction of starlight. As the stars are very far away they behave as almost point sources of light. On account of atmospheric refraction, 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. As a result of it, sometimes, the star appears brighter and at some other time, fainter. Thus the star twinkle.

A person is said to have a myopia vision; of he can see objects situated near the eye clearly but cannot clearly see distant objects. If a person can see clearly upto a distance ‘x’ from the eye, then it means that far point of eye has shifted from infinity to a point O situated at distance ’x’ from the eye.
Two possible causes of myopia are:
(i) Either the power of the eye lens has become more than its normal value due to excessive curvature of the cornea (focal length of eye lens has decreased).
(ii) Elongation of the eye ball due to some genetic defect.
In the given problems as the boy is using spectacles of focal length f=-50cm. It means he is using concave lens and hence he is suffering from myopia.

(a) The student is suffering from myopia.
Two possible causes of myopia are:
(i) Either the power of the eye lens has become more than its normal value due to excessive curvature of the cornea (focal length of eye lens has decreased).
(ii) Elongation of the eye ball due to some genetic defect.
(b) If an eye donation camp is being organized by social workers in our locality we should whole heartedly. Participate in the camp. We should fill up forms pledging to donate our eyes after our death. We should also encourage our family members, friends and others to pledge eye donation.
It is because our eye donation could give new light to a number of blind persons. In this way our eyes can see the world even after our death. Eye donation is a noble cause and we should propagate the idea.

Long sightedness or far sightedness or hypermetropia is that defect of vision in which the affected person can see distant objects clearly but cannot see nearby objects so clearly. The eye lens can focus the rays at point I on retina for objects situated at a distance x (where x> D) or more from the eye. It means that far point of defective eye is at infinity but near point of defective eye has shifted away to N’.
For objects situated at least distance of distinct vision D (i.e., at point N) the light rays are focused at a point I1 behind the retina.
Two possible causes, of hypermetropia are:
(i) Either the power of the eye lens is less (or focal length of eye lens is too long) due to less curvature of cornea.
(ii) Shortening of eye ball due to some genetic defect.
To rectify this defect, we use a convex lens L1 of focal length f so as to form virtual image of an object situated at the least distance of distinct vision (u=-D) at the near point N’ of the defective eye (v=-x) so that now the eye lens can focus the rays so as to form a sharp image I at the retina.

(a) A pencil ABC partly immersed in water appears to be bent at the water surface. It is on account of bending of light ray away from the nor mal when refraction of light take place from water (optically denser medium) to air (optically rarer medium).
(b) At noon, the sunlight would travel relatively shorter distances before reaching us. As a result, scattering of blue and violet light is very less and the sun appears white.
In early morning or evening, Sun is near the horizon and sunlight passes a longer distance in earth’s atmosphere before reaching us. So most of the blue-violet light is scattered away. It gives reddish appearance to the sun.