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Q. 9. The stars appear to twinkle while planets do not. Why? 2

Ans. The stars appear very-very small as they are far away so stars can be considered to be point sources of light. The continuously changing atmosphere causes variations in the light coming from the point sized stars due to refraction and thus the stars appear to be twinkling. On the other hand, the planets appear to be quite big as they are much nearer to the earth. So a planet can be considered to be a collection of a very large number of point sources of light. The dimming effect produced by some of the point sources of light in one part of the planet is nullified by the brighter effect produced by the point sources of light in its other part. Thus the brightness of a planet always remains the same and hence it does not appear to twinkle.

Q. 10. Why does the sky look blue on a clear sunny day? Explain. 2

Ans. See Q.18 (S.A.T), Chapter 11.

Q. 11. Draw a circuit diagram to show an electric lamp of resistance 60  and a resistor of 12 connected in series with a battery of 12 V. Calculate.
(a) total resistance in the circuit
(b) current flowing through the circuit
(c) potential difference separately across the lamp and the resistor. 3

Sol.

Science 2009 Question Papers Class X
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	Set 1 (PDF)		Set 1 (PDF)		Set 1		Set 1		Set 1 (PDF)
					Set 2		Set 2
					Set 3		Set 3