CBSE Class 11-science Answered
A real image 2 times the size of an object is formed by a concave mirror. When the mirror and the object are shifted, the image formed becomes 4 times the size of the object. Find the focal length of the mirror if the image were to be shifted 24cm
Asked by bukokaakinsola39 | 31 May, 2019, 08:56: AM
if magnification is 2, then we have 2 = | v1 / u1 | or u1 = v1 / 2 , where v1 is mirror-to-image distance
and u1 is object-to-mirror distance.
hence by mirror equation, we have ( 1/v1 ) + ( 1/u1) = 1/f or ( 1/v1 ) + ( 2/v1) = 3/v1 = 1/f or v1 = 3f ....................(2)
where f is focal length of mirror
Similarly in second case, if magnification is 4, then we have 2 = | v2 / u2 | or u2 = v2 / 2
and ( 1/v2 ) + ( 4 / v2 ) = 5/v2 = 1/f or v2 = 5f ....................(3)
Hence shift in image , v2 - v1 = 5f-3f = 2f = 24 cm or f = 12 cm
Answered by Thiyagarajan K | 31 May, 2019, 09:41: AM
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