Question

a concave mirror has a focal length of 40 cm . determine the position of the object for which an erect and four times the size of the object image is formed
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Answer 1

height of image = 4x coz let height of object = x

magnificattion = h2 / h1 = -v/u

ie -v/u = 4 therefore v = -4u

mirror formula

1/v + 1/u = 1/f

1/-4u + 1/u = 1/-40

1/u - 1/4u = -1/40

4u-u/4u2 = -1/40

(canceling 40 by 4 to get 10)

3u/u2 = -1/10

(cancelling u by u)

3/u = -1/10

therefore u =-30cm

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