Question

Explain the behaviour of light rays in any four situations of their incidence on a convex lens.

Answer 1

Convex lens is a converging lens. Convex lens generally forms real inverted images of the object except in one case when the object is kept between the focus and the pole of the lens. The size of the image formed by a convex lens keeps on increasing as the object advances towards the pole of the lens.

Complete step by step answer:
Since we have to show the behaviour of light rays, we’ll consider light rays falling on the lens in four different scenarios. We’ll consider light rays parallel to the principal axis, light rays passing through the focus, light rays incident at the pole of the lens and light rays appearing to converge at the focus. We’ll take these cases one by one.

1. When light rays are parallel to the principal axis:
   When light rays parallel to the principal axis are incident on the convex lens, they are converged at the focus. This can be demonstrated using the following figure. Since the rays converge, the image formed is point-size, which is highly diminished.
   

2. When light rays passing through focus are incident on the lens:
   In this case, the light after an incident on the lens becomes parallel to the principal axis.
   

3. When light rays are incident at the pole of the lens:
   Light rays do not suffer any deviation when passing through the pole of the lens, that is, they pass through as they were incident.
   

4. When light rays appear to converge at the focus:
   This situation arises when light rays are not passing through the focus but they only appear to converge at the focus. The light rays become parallel to the principal axis after refraction through the lens.
   

Note: Generally, by convex lens, we mean a biconvex lens, in which both the refracting surfaces are convex. But we also have a Plano-convex lens where one refracting surface is plane and the other is convex and a Concavo-convex lens where one refracting surface is concave and the other is convex. The same rules apply to all of the above lenses as long as the refracting surface is convex.