Attempt either (a) or (b) : (a) How does the focal length of eye lens change as the distance of the object from the eyes is altered ? Explain.
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(a) * If the distance of the object from the eye is increased, ciliary muscles relax, lens becomes thin and so the focal length increases (1 Mark) * If the distance between the object from the eye is decreased, the ciliary muscles contract, lens becomes thick and so the focal length decreases. (1 Mark) OR (b) In myopic eye image is formed in front of the retina. A concave lens / diverging lens of suitable power will bring the image back on to the retina. (2 Marks)
(i) How does the change in curvature of the eye lens helps us in the process of seeing the nearby objects clearly ? (ii) State the range of the power of accommodation of a normal human eye.
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(i) While looking at objects nearer to eye, the curvature of the eye lens increases. Consequently, the focal length of the eye lens decreases. (1 Mark) (ii) From 25cm (near point) to infinity (far point) (1 Mark)
(b) Why is the concave lens used as a corrective lens for a myopic eye ?
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(a) * If the distance of the object from the eye is increased, ciliary muscles relax, lens becomes thin and so the focal length increases (1 Mark) * If the distance between the object from the eye is decreased, the ciliary muscles contract, lens becomes thick and so the focal length decreases. (1 Mark) OR (b) In myopic eye image is formed in front of the retina. A concave lens / diverging lens of suitable power will bring the image back on to the retina. (2 Marks)
(A) The elderly people, may have both near sightedness and far sightedness. Write the possible solutions, can be proposed for correction of their eye defect. (B) How does the eye control the amount of light entering into it ?
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(A) (i) They either use two sets of spectacles one for correcting Near sightedness and other for correcting farsightedness (1 Mark) (ii) Use bifocal spectacles / use progressive lenses. (1 Mark) (B) The pupil regulates and controls the amount of light entering the eye. The size of pupil is regulated by Iris (a muscular diaphragm). (1 Mark)
Two persons, A and B are using lenses of power $+1.5\ \text{D}$ and $-1.5\ \text{D}$ respectively in their spectacles. Write the names of eye defects from which A and B are suffering. Write two causes for each defect of vision.
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• A is suffering from Hypermetropia ($\frac{1}{2}$ Mark) • B is suffering from Myopia ($\frac{1}{2}$ Mark) Causes of Hypermetropia (A) (i) Focal length of eye lens is too long ($\frac{1}{2}$ Mark) (ii) Eye ball has become too small ($\frac{1}{2}$ Mark) Causes of Myopia (B) (i) Excessive curvature of eye lens ($\frac{1}{2}$ Mark) (ii) Elongation of eye ball ($\frac{1}{2}$ Mark) (3 Marks)
A thin beam of white light falls on a face of a glass prism and a spectrum is obtained on a screen. Which of the following options is the correct sequence of colours from top to bottom on the screen ?
Attempt either subpart (a) or (b) : (a) Name the phenomenon due to which a prism splits the incident white light into a band of colours. State the reason of the phenomenon also.
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(a) • Dispersion of light (1 Mark) • Different colours of light bend through different angle with respect to incident ray as they pass through prism. (1 Mark)
(a) Name a phenomenon taking place in nature where light rays are dispersed. When, where and how does this phenomenon happen?
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(a) • Rainbow formation ($\frac{1}{2}$ Mark) • After the rain shower ($\frac{1}{2}$ Mark) • In the sky in a direction opposite to that of Sun ($\frac{1}{2}$ Mark) • Due to refraction, dispersion and internal reflection of sunlight through droplets of water suspended in air. ($\frac{1}{2}$ Mark) (2 Marks)
Explain the phenomenon of rainbow formation in the sky. Draw the labelled ray diagram in support of your explanation.
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Rainbow is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. 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.
What is meant by atmospheric refraction of light ? Explain the following natural phenomenon by using the concept of atmospheric refraction : (a) Twinkling of stars (b) Flickering of objects seen through a turbulent stream of hot air
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$\bullet$ Refraction of light by the earth's atmosphere due to variation in optical density of different layers of air is called as atmospheric refraction. (1) (a) The starlight, on entering the earth's atmosphere undergoes refraction continuously in a medium of gradually increasing refractive index and we see the apparent position of the star. Since the physical conditions of atmosphere are not stationary, the apparent position of the star fluctuates and gives twinkling effect. (1) (b) The air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dens) than cooler air above it and has a refractive index slightly less than that of cooler air. Since the physical conditions of refracting medium are not stationary, the apparent position of objects, as seen through hot air fluctuates. And we see flickering objects. (1)
Attempt either (a) or (b) : (a) What happens when light is scattered from (i) very fine particles and (ii) very large size particles of medium ? Justify your answer in each case.
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(a) (i) Mainly blue coloured light is scattered because very fine particles scatter shorter wavelength. (1) (ii) Scattered light may appear white because very large size particles scatter light of all wavelengths. (1)
What is meant by scattering of light ? How do our observations differ when white light (sun light) is scattered from (i) very fine particles of the medium and (ii) very large particles of the medium ?
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The phenomenon in which a part of light incident on a particle is redirected in different directions is called scattering of light. (1 Mark) (i) Shorter wavelength (blue light) is scattered. ($\frac{1}{2}$ Mark) (ii) White light is scattered. ($\frac{1}{2}$ Mark)
(a) What is Tyndall effect ? (b) What happens when sunlight is scattered from the particles of very large size ? (c) ‘Danger’ signals are always red in colour. Why ?
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Red colour is least scattered by fog or smoke, so it can be seen in the same colour from a large distance. (1 Mark)
What is scattering of light ? How is scattering of light different from reflection of light? Explain.
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* The phenomenon of spreading light in different directions on interaction with particles of the medium. (1 Mark) * Light bounces back in a fixed direction after reflection while in scattering of light, it spreads in different directions. /Reflection of light is independent of the size of reflecting particles whereas the colour of the scattered light depends upon the size of scattering particles (2 Marks)
(a) Explain the Tyndall effect with the help of one suitable example. (b) What happens when a white light gets scattered from (i) very fine particles (ii) larger size particles ?
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(a) • When a beam of light strikes fine particles like smoke, tiny water droplets, suspended particles of dust and molecules of air, the path of beam becomes visible. This phenomenon of scattering of light by colloidal particles is called Tyndall effect. (1 Mark) • Examples: - ➤ When sunlight passes through a canopy of a dense forest tiny water droplets in the mist scatter light. (1 Mark) ➤ When a fine beam of sunlight enters a smoke-filled room through a small hole, scattering of light makes the particles visible. (Any one, any other suitable example) (b) (i) Blue light / light of shorter wavelength is visible. (½ Mark) (ii) Red light/ Light of longer wavelength is visible. (½ Mark)
(a) What is meant by Tyndall effect ? (b) How is scattering of light different from reflection of light ? (c) What will happen if white light (sunlight) is scattered from (i) very small particles of medium, and (ii) very large particles of medium ?
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(a) Tyndall Effect :- The phenomenon of scattering of light by the colloidal particles which makes the path of light visible. This is called Tyndall effect. (1) (b) Scattering of light depends on size of particles of colloid and wave length of colour of light while reflection of light is independent of both of them. (1) (c) (i) Scatter the Short wavelength colours of sunlight (½) (ii) White light scattered (½)