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1. An object of size 9 cm is placed at 20 cm in front of a concave mirror of focal length 15 cm. At what distance from the mirror should a screen be placed, so that a sharp focussedimage can be obtained? Find the size and the nature of the image. |
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Answer» wer : POSITION of object = 9cm Distance of object = 20cm Focal length = 15cm Type of mirror : concave We have to find position of object and height of image ★ Focal length of concave mirror is taken negative and that of convex mirror is taken positive. Position of image can be measured by using mirror formula.
u denotes position of object v denotes position of image f denotes focal length By SUBSTITUTING the values, we get ➝ 1/(-20) + 1/v = 1/(-15) ➝ 1/v = -1/15 + 1/20 ➝ 1/v = (-4 + 3)/60 ➝ v = -60 cm There for screen should be placed at 60cm in front of mirror in order to get SHARP focussed image. Formula of magnification is given by ➝ m = -v/u = h'/h
➝ -(-60)/(-15) = h'/9 ➝ -4 = h'/9 ➝ h' = -36 cm Negative sign indicates that image is REAL and inverted. Nature : Real, inverted and enlarged |
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