Question

Define the principal focus of a concave and convex lens.

Answer 1

1. The principal focus of a Convex lens:

1. A convex lens is a lens that is thick at the middle and thin at the edges.
2. When the incident rays are parallel to the principal axis, they converge at a point after refraction.
3. This point where the parallel set of incident rays meet at a point after refraction is called the principal focus of a convex lens.
4. Since the rays actually meet at focus in a convex lens, it is said to be a real focus.

2. Principal focus of Concave Mirror :

1. A concave lens is a lens that is thin at the middle and thick at the edges.
2. When the incident rays are parallel to the principal axis of a concave lens, they diverge.
3. When the refracted rays are produced backward, they appear to diverge from a single point. This point from where the refracted rays appear to diverge is called the principal focus of a concave lens.
4. The principal focus of a concave lens is said to be a virtual focus because it does not really exist, it only appears to exist when rays are produced backward.