real image formed by a concave mirror is 4.5 times the size of an object if the if the mirror is 20 centimetre from the object its focal length is ________
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Explanation:
magnification m = - v / u = -4 (given)
v is mirror to image distance; u is mirror to object distance = -20 (by sign convention)
hence v = -80 cm
we have mirror equation, begin mathsize 12px style 1 over v space plus space 1 over u space equals space 1 over f end style
hence (-1/80)+(-1/20) = (-5/80) , f = -16 cm
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