A convex lens is placed in contact with a plane mirror. An axial point object at a distance of `20 cm` from this combination, has its image coinciding with itself. What is the focal length of the convex lens ?

Explanation

Here, the convex lens is in contact with a plane mirror and the image distance is equal to the object distance. This is possible only when the point object is placed at the centre of the curvature of the lens.
We use the relation `F=(R)/(2)` where R is the distance between the centre of the curvature and the pole and f is the focal length.
Here, R = 20 cm
`therefore `Focal length of the lens = 20/2 = 10cm.