Question

A convex lens and a convex mirror are separated by a distance of 10 cm such that the reflecting surface of the mirror faces the lens. The image of an object placed in front of the convex. Lens at a distance of 20 cm is found to coincide with the object. If the focal length of the convex lens is 15 cm, determine the focal length of the mierror.

Answer 1

If the mirror were not present
` 1/f = 1/v - 1/u`
` 1/15 = 1/v - (-1/20)`
` 1/v = 1/15 - 1/20 `
` (20-15)/300 , v = 60` cm

The image would be formed at a distance of 50 cm from the mirror.
When the mirror is placed the image is formed as the position of the object.
That is the rays are reflected back along the same path.
This is possible only when the light rays are incident normally.
` :. 10 + R = 60` cm
R = 50 cm
f = 25 cm