Class 10 Science Chapter 10 Important Questions

Pupil control the amount of light entering the eye. Iris control the size of the pupil and hence regulates the amount of light entering the eye.

Blind spot is the region of the retina where the optic nerve enters the eye is called blind spot. Yellow spot is the region in the center of the retina which has maximum concentration of rods and cones.

(i) Pupil
(ii) Retina

Point of similarity
Camera:
1. Image is formed by a convex lens made of glass.
2. A real and inverted image is formed on the photographic film.
3. Diaphragm controls the amount of light entering in the camera.
4. Time of exposure is controlled by a Shutter.
Human Eye: 1. Image is formed by the eye lens made of fibrous, jelly like material.
2. A real and inverted image formed on the retina.
3. Pupil in the iris controls the amount of light entering the eye.
4. Time of exposure is controlled by the eyelids.
Point of Dissimilarity
Camera:
1. Focal length of camera lens is fixed.
2. Focussing is done by changing the distance between the camera lens and the photographic film.
3. Photographic film retains the image permanently.
4. A photograph has to be changed for getting next photograph.
5. The angular region is covered is about 600
Human Eye:
1. Focal length of eye lens can be changed with the help of the ciliary muscles.
2. Focussing is done by changing the shape of eye lens by the action of ciliary muscles.
3. The retina of the eye retains the impression of an image for about (1/16)th of a second.
4. The same retina can be used for viewing an unlimited number of image.
5. The angular region covered in about 1500.

Accommodation: Accommodation is the ability or property of eye lens due to which it can change its curvature r focal length so that image of objects at various distances can be formed on the same retina. The focal length of the eye lens is automatically changed with the help of ciliary muscles as follow:
(i) Viewing far off Objects: When the ciliary muscles are completely relaxed the eye lens is thin and its focal length is maximum(equally to distance between eye lens and retina). The rays coming from the distance object are parallel to each other and they are focused at retina.
(ii) Viewing nearby objects: When we look at a nearby objects the ciliary muscles contract the eye lens bulges out and became thick and its focal length is reduced. This focusses the light from the nearby object on the retina.

The minimum distance from the eye at which the eye can see the objects clearly and distinctly without any strain is called the distance of distinct vision. For a normal eye its value is 25 cm.

In old age the crystalline lens of some people becomes hazy or even opaque due to the development of membrane over it. This condition is called cataract. This causes a decreases or loos of vision of the eye. The vision can be restored after getting cataract surgery.

Advantages of binocular vision: There are many advantages of having two eyes instead of one. These are as follows:
1. It gives a wider horizontal field of view of about 1800.
2. It helps to detect even fainter objects.
3. It provides a three dimensional of objects around us.
4. The two eyes give relief to one another after every fraction of a second.

Defects of vision: A normal eye can see objects clearly at any distance between 25 cm and infinity from the eye. Sometimes, a human eye gradually loses its power of accommodation. The accommodation. Then we cannot see the objects clearly. Our vision becomes defective. There glasses are mainly for common defects of vision which can be corrected by the use of suitable eye glasses. These defects are:
(i) Myopia or near sightedness
(ii) Hypermetropia or far-sightedness
(iii) Presbyopia
(iv) Astigmatism

Myopia or short sightedness: It is a vision defect in which a person can see nearby objects clearly but cannot see the distant objects clearly beyond a certain point. This defect is common among children.
Cause of myopia: This defect arises due to either of the following two reasons:
(i) The eyeball gets elongated along its axis so that the distance between the eye lens and the retina become larger.
(ii) The focal length of the eye lens becomes too short due to the excessive curvature of cornea.
Correction of Myopia: A myopic eye is corrected by using a concave lens of focal length equal to the distance of the far point F from the eye. This lens diverges the parallel rays from distant object as if they are coming from the far point F. Finally, the eye lens forms a clear image at the retina.

Calculation of focal length and power of correcting lens in myopic: Let x be the distance of the actual far point from the eye and hence from the concave lens placed close to the eye. The rays coming from infinity after refraction through the concave lens, appear to come from the far point F.
U= -∞ , V=-x, f=?
By lens formula,
1/f= 1/v – 1/u = 1/(-x) – 1/(-∞) = 1/x + 0 = -1/x
Required focal length f=-x
Required power = 1/f = -1/x
The negative sign shows that the correcting lens is concave.

(a) Hypermetropia or long sightedness: It is a vision defect in which a person can see the distant objects clearly but cannot see the nearby objects clearly.
(b) Cause of hypermetropia: This defect arise due to either of the following two reason.
(i) The eye ball become two long its axis so that the distance between the eye lens and the retina is reduced.
(ii) The focal length of the eye lens becomes too large resulting in the low conversing power of the eye lens.
(c) To focus the rays again on the retina the object has to be moved away from the eyes to a distance greater than 25cm thus the near point of the eye is not at 25cm but it has shifted to N at a distance greater than 25cm from the eyes.
Correction of hypermetropia: A hypermetropia eye is corrected by using a convex lens of suitable focal length. This lens diverges the rays such that the rays coming from normal near point N appear to come after refraction from near point N of the defected eye. That is the virtual image of the object place at N is formed at N1. Then the eye lens forms a clear image at the retina.

Presbyopia: This defect is similar to hypermetropia that why a person having this defect cannot see nearby objects distinctly but can see distant objects without any difficulty. This defect differs from hypermetropia in the cause by which it is produced. It usually occurs in elderly persons. Due to the stiffening of the ciliary muscles, the eyes loses flexibility and hence the accommodating power of the eye lens decreases.

Some people suffer from both myopia and hypermetropia. Such people require bi-focal lenses. The upper part of the bi-focal lens is a concave lens used for distant vision while its lower part is a convex lens used for reading purposes.
These days’ refractive defects are also corrected by using contact lenses or through surgical interventions.

(a) The person is suffering from both myopia and hypermetropia.
(b) The person requires bi-focal lenses to increases his range of vision. The upper part of the bi-focal len is a concave lens which facilitates distant vision while the lower pat is a convex lens which facilitates near vision.

It is defect of vision in which a person cannot distinguish between various colous but can see well otherwise.

It is due to absence of some cones in the retina of the eye.

The phenomenon of the continuation of the impression of an image on the retina for some time even after light from the object is cut off is called persistence of vision.

Prism: A prism is a portion of a transparent medium bounded by two plane faces inclined to each other at a certain angle.
The two plane faces ABED and ACFD inclined to each other are called refracting faces of the prism.
The line AD along which the two refracting faces meet is called refracting edge of the prism.
The faces BCFE opposite to the refracting edge is called base of the prism.
The angle A at which the two refracting faces are inclined to each other is called angle of the prism.
A section ABC made by a plane at right angles to the refracting edge is called principal section of the prism.

(a)Dispersion: When a narrow beam of white light (sunlight or touch light) is passed through a triangular glass prism, it splits into a band of seven colours. The seven colours are in the order violet, indigo, blue, green, yellow, orange and red. The red colour is deviated the least while the violet colour is deviated the most. The colour sequence can be remembered by the acronym VIBGYOR.
The phenomenon of splitting of white light into its component colours on passing through a refracting medium such as a glass prism is called dispersion of light. The pattern of the coloured bands obtained on the screen is called spectrum.
Sir Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight.
(b) Cause of dispersion of while light: Light rays of different colours travel with the same speed in vacuum. But in refracting media like glass, water, etc., lights of different colours travel with different speeds. The speed of violet colour is the least while the speed of red colour is largest in glass. As a result the refraction index of glass is largest for violet colour and least for red colour. The violet colour is deviated most while the red colour is deviated least on passing through the prism. Other colours are deviated by intermediate angles. So, different component colours of white light get dispersed on passing through a glass prism.

Recombination of seven colours to form white light: Take two identical prisms P1 and P2. Keep the second prism P2 upside down with respect to the first prism. Allow a narrow beam of white light to fall on first prism P1.
The first prism P1 disperses the white light into seven colours. The second prism P2 receive these seven colours and recombines them into white light. Thus, a beam of white light emerges from the other side of the second prism. This observation gave Newton the idea that the sunlight is made the seven colours.
Any light that gives a spectrum similar to that of sunlight is called white light.

(a) The light of single wavelength or single colour is called monochromatic light. For example, the yellow light f sodium lamp is almost monochromatic.

(b) The light that consists of many wavelengths or many colours is called polychromatic light. The white light of the sun or that of an electric bulb is polychromatic.

Rainbow: It is a beautiful spectrum seen in the sky when the sun shines on raindrops during or after a rain shower. The water droplets act like 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 the dispersion and internal reflection of light, different colours reach the observer’s eye along different pairs. An observer standing with his back towards the sun observes the rainbow in the form of concentric circular arcs of different colours in the horizon.

(a) Refraction
(b) Dispersion
(c) Internal reflection of light.

Twinkling of stars: The apparent position of a star is slightly different from the actual position due to refraction of starlight by the atmosphere. Further, this apparent position is not stationary but keeps on changing due to the change in atmospheric conditions like density, temperature, etc. The path of the rays of light coming from the star goes on varying slightly. The amount of light entering our eyes from a particular star increases or decreases randomly with time. Sometimes, the star appears bright and other times, it appears fainter. This gives rise to the twinkling effect of the star.
The planets do not show twinkling effect: As the planets are much closer to the earth, the amount of light received from them is much greater and the functional caused in the amount of light due to atmospheric refraction are negligible as compared to the amount of light received from them.

Red, Orange, Yellow, Green, Blue, Indigo and Violet.

Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The stars appear slightly higher (above) than their actual position when viewed near the horizon.

Advance sunrise and delayed sunset. Apparent shift the position of sun at sunrise and sunset: The sun is visible before actual sunrise and after actual sunset, because of atmospheric refraction. With altitude the density and hence refractions index of air layer decreases. The light rays starting from the sun travel from rarer to denser layers. They bend more and more towards the normal. To an observer on the earth, light rays appears to come from position S’. The sun which is actually in position S, below the horizon appears in positions S’ above the horizon. Thus the sun appears to rise early by about two minutes and for the same reason, it appears to set late by about two minutes. This increases the length of the day by about four minutes.

Apparent flattening of the refractive index of the atmosphere decreases with altitude, so the rays from the top and bottom formation of the sun on the horizon are refracted by different degrees. This causes the apparent flattering of the sun. But the rays from the sides of the on a horizontal plane are generally refracted by the same amount so the sun still appear circular along its sides.

Colloids: A colloid is an intermediate state of true solution and suspension. A colloid is a heterogeneous system in which one substance called dispersion medium. Colloidal particles have sizes between 1 and 1000 nm. For example, in milk water is dispersion medium while fats, proteins etc., constitute disperse phase.
Properties of colloidal solutions:
1. They are heterogeneous mixtures.
2. The size of the colloidal particles ranges between 1 nm to 1000nm.
3. The path of a beam of light becomes visible while passing through a colloidal solution.
4. Colloidal particles cannot be separated by simply sedimentation or filtration.

Tyndall effect: When a beam of light is passed through a colloidal solution placed in a dark room the path of beam becomes illuminated when observed through a microscope placed perpendicular to the path of light. This effect is called Tyndall effect.
Cause of Tyndall effect: The size of the colloidal particles is relatively larger than the solute particle of a true solution. The colloids particles first absorb energy from the incident light and then scatter a part of this energy from their surfaces. Thus Tyndall effect is due to scattering of light by the colloids particles and the colloids particles are seen as points of light moving against a dark background.
Some daily life phenomena based on Tyndall effect are as follow:
1. When a fine beam of sunlight enters a smoke filled room through a small hole, the smoke particles become visible due to the scattering of light.
2. When sunlight passes through a canopy of a dense forest, the tiny water droplets in the mist scatter light and become visible.

The colour of the scattered light depends on the size of the scattering particles. Very fine particles having sizes less than the wavelength of visible light mainly scatter blue light of shorter wavelength of the visible spectrum. The particles of relatively larger size scatter light of longer wavelengths. The larger particles like raindrops, dust and ice particles scatter white light.

Blue of the clear sky: The blue colour of the sky is due to the scattering of the sunlight by the molecules of the atmosphere. The molecules of air such as N2 and O2 have sizes smaller than the wavelength of visible light. As sunlight passes through the atmosphere, these molecules absorb some amount of sunlight and remit it. They scatter blue light of shorter wavelength more strongly than red light of longer wavelength. When we look at the sky the scatter light enters our eyes and this light contains blue light in a larger portion that’s why the shy appears blue.

In the absence of any atmosphere there will be no scattering of sunlight and the sky appear dark. The sky appears dark to passenger or astronaut flying at high altitudes as scattering is not prominent at such height due to thin atmosphere.

The sky appears dark from the surface of the moon because there are no atmospheric particles to scatter sunlight.

In the visible spectrum the rod colour has the largest wavelength. The red colour is least scattered by fog or dust particles. So we can observe red colour easily even in foggy and dusty conditions.

The water molecules of the ocean scatter blue light more strongly than light of other colours. So the ocean appears bluish.

Reddishness at sunset and sunrise: When the sun is near horizon at sunset or sunrise the light rays have to pass through a larger thickness of the atmosphere than when the sun is overheated at noon. Consequently, the lower wavelengths in the blue region are almost completely scattered away by the air molecules. The higher wavelengths of the red region are least scattered and reach our eyes. Hence the sun appears almost reddish at sunset or sunrise.

The sun appears reddish at sunset/sunrises while it appears white at moon. At sunset/sunrise the sun is near the horizon. The larger thickness of the atmospheric molecules scatters most of the lower wavelength of blue region and so the sun appears red. At noon, the light from the sun overhead travels relatively a smaller thickness of the atmosphere. So the sun appears white as only a little of the blue light and shorter wavelengths and scattered away by the atmospheric molecules.

(i) Stars appear to twinkle due to atmospheric refraction of starlight and physical conditions of the earth atmosphere is not being stationary.
(ii) Planets are much closer to the earth and are seen as extended sources. The fluctuation caused in the amount of light due to atmospheric refraction are negligible Hence, planets do not twinkle.

A concave lens of suitable focal length diverges the parallel rays from the distant object as if they are if they are coming from the far point F of the myopic eye. This helps the eyelens to form a clear image at the retina.

A convex lens of suitable focal length converges the rays such that the rays coming from the normal near point N appear to come after refraction from the near point N of the defected eye. Then the eye lens forms a clear image at the retina.

The remedial lens should make the object at infinity appear at the far point.
For object at infinity, u= -∞
Far point distance of the defected eye, v= -150 cm
By lens formula,
1/f = 1/v – 1/u = 1/(-150) – 1/(-∞) = – 1/150 + 0 = -1/150
f = -150 cm
P = 1/(f(in m)) = 1/(-1.50 m) = -0.67 D.
Negative sign shows that the remedial lens is a concave lens.

Clearly the lens used in this case should be such that the rays proceeding from a point distant 12 m from the eye should after refraction appears to come from a point distant 3 m from the eye.
Here u = -12 m, v = -3 m, f =?
Using lens formula,
1/f = 1/v – 1/u = 1/3 + 1/12 = -1/4 or f = -4 m
Thus a concave lens of focal length 4 m should be used.

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.

Planets are much closer to the earth and are seen as extended light source, So a planet may be considered as a collection of a large number of point sized light sources. Although light coming from individual point-sized sources flickers but the total amount of light entering our eye from all the individual point sized sources average out to be constant. Thereby planets appear equally bright and there is no twinkle of planets.

In the early morning the Sun is situated near the horizon. Light from the sun passes through thicker layer of air and covers a larger distance in the earth’s atmosphere before reaching our eyes. While passing through the atmosphere blue light is mostly scattered away and sun appear reddish.

Blue colour of the sky is on account of scattering of light o shorter wavelengths by particles in the atmosphere of earth. If the earth has no atmosphere there would not have been any scattering and the sky would have looked dark. When an astronaut in his spacecraft goes above the atmosphere of earth sky appear dark to him because there is no scattering of light.

The power of accommodation of the eye is the maximum variation of its power for focusing on near and far objects. For a normal eye, the power of accommodation is about four Diopters.