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.

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

myth. it was once thought that animals, including cats and dogs, could only see in black and white. however, scientists have proven this to be a myth. in all animals, including humans, the perception of colour is determined by the presence of cells in the eye called cone photoreceptors.