Question

The focal length of a concave mirror is 18cm at what distance should it be placed to get a 3 times magnified image

Answer 1

The screen should be placed at a distance of 54 cm on the object side of the mirror to obtain a sharp image. The image is real, inverted and enlarged in size.

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Answer 2

Explanation:

given that

f = - 18cm

m = 3

u = ?

by mirrors formula

1/f = 1/v + 1/ u

- 1/18 = 1/v + 1/ u ---- eqn 1

m = - v / u

3 = - v / u

- v = 3u

v = - 3u ------- eqn 2

put the value of v in eqn 1 we get

-1/18 = -1/3u + 1/u

-1/18 = ( -1 + 3) / 3u

-1/18 = 2/ 3 u

-3u = 18 x 2

-u = (18 x 2 ) / 3

-u = 12 cm

u = -12 cm

put the value of u in eqn 2

v = -3u

v = - 3 x -12 = 36 cm

screen should be placed at the distance of 12 cm from the mirror to get 3 times magnified image