Question

A concave mirror of focal length 30cm forms a image of an object, twice the size of object. Find the position of the object.

Answer 1

given,

focal length = - 30cm

let object be x and image be 2x

according to magnification formula

m =

height of image/height of object

m = 2x/x

m = 2

according to magnification formula

m = -v/u

2 = -v/u

2u = -v

v = -2u

according to mirror formula

1/f = 1/v + 1/u

-1/30 = -1/2u + 1/u

-1/30 = -1+2/2u

-1/30 = 1/2u

-2u = 30

u = 30/-2

u = -15

hence image formed 15cm away from mirror

here negative sign indicates that image is real

image formed is between p and f

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