Question

what is magnification and focal length of a plane mirror ?

Answer 1

The focal length of a plane mirror is infinity. So, 1/f = 0

Using,

1/f = 1/v + 1/u

=> 0 = 1/v + 1/u

Applying sign convention, ‘u’ is negative, ‘v’ is positive and ‘f’ is also negative.

Thus,

0 = 1/v – 1/u

=> v = u

Magnification of mirror is given by, m = v/u

So, m = u/u = +1

The ‘positive’ magnification indicates virtual image. And ‘1’ implies the size of object and image are same.