Question

at what distance of plane mirror of focal length2.5m should a boy stand so that his image has height equal to half of original height

Answer 1

Let the actual height be x

=(x/2)/x

=1/2

1/2= -v/u

Using sign convention,

1/2 = v/u

u=2v

That is object distance is twice the image distance

So, the object should be placed at the centre of curvature and the image will be formed at focus.

Object should be placed at centre of curvature =2(2.5)=5cm

The image will be formed at focus =2.5cm