Question

a concave mirror produces three times enlarged image of an object placed at 10cm in front of it . calculate the focal length of the mirror?

Answer 1

here, magnification is

m = -v/u = hi/ho = 3

so,

v = -3u

the focal length would be

1/f = 1/v + 1/u

or

1/f = 1/u - 1/3u

or

1/f = 2/3u

thus,

f = 3u/2

here u = 10 cm

so, the focal length would be

f = 30/2 = 15 cm