Question

A. Explain with the help of a diagram, why the concave lens is also called a diverging lens.
B. Define the principal focus of a concave lens.

Answer 1

Hint: We can deduce that the term diverging refers to incident rays diverging away from a point. As a result, we'll start by looking at how incident rays behave when they strike a concave lens. We'll also evaluate if the concave lens is a converging or diverging lens based on the behaviour of the refracted rays.

Complete answer:
A. To understand let us first draw the ray diagram first of a biconcave thin lens and when the incident rays coming from infinity and focused at the point $F$ focus on the principal axis, we see that:

The incident rays diverge following refraction by the concave lens, as shown in the diagram above. These rays appear to originate from a point F, i.e., the concave lens's focus. Because of the geometry of the aforementioned lens, the rays diverge. We can deduce that the concave lens is a diverging lens since these rays diverge from a point on the primary axis, which is the focus of the concave lens.

B. A point on a concave lens's principal axis from which light rays that were previously parallel to the axis appear to deviate after passing through the lens is called the principal focus.

Note: Due to its feature, the concave lens has a wide range of applications. The image is created in front of the retina in those who suffer from myopia. As a result, the optician will advise the person to wear spectacles with a concave lens, which will diverge the rays from the image formation point to the retina of the eye. Because the focal length of a concave lens is negative, its power is always regarded as negative.