Question

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

Answer 1

The focal length of the lens is 10 cm.

Step-by-step explanation:

The question involves understanding the properties of a convex lens when combined with a plane mirror. When an object is placed at a certain distance from this combination and its image coincides with itself, it means that the image formed by the lens is reflected back to the same point by the plane mirror. In this scenario, the object distance (do) is equal to the image distance (di), which is 20 cm. To find the focal length (f) of the convex lens, we use the lens formula which is 1/f = 1/do + 1/di.

Considering that do is equal to di, the equation becomes 1/f = 1/20 + 1/20 = 2/20. Solving for f, we get f = 20/2 = 10 cm. So, the focal length of the lens is 10 cm.