In the autocollimation experiment of sec. 3.5, a thin convex lens is placed 25 cm from a pinhole that acts as a point source of light. a plane mirror placed 20 cm behind the lens reflects the light back through the lens, and the reflected light forms a sharp spot beside the pinhole. what is the focal length of the lens (in cm)?

25 cm

Explanation:

Since, in the question it is given that the final image after reflection through the convex lens and then the reflection by the plane mirror is formed at the pinhole itself.

This means that the pinhole must be at the focus of the convex lens, such that the lens makes the ray perpendicular to the mirror and hence focal length of the lens = 25 cm.

there are no contact forces mentioned on the list of choices that you've provided.