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 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)
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.
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)
(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)