Question

How many foci does a lens have?

Answer 1

Foci of a Lens:

1. When light rays come from infinity or are incident parallel on a lens, they refract and actually meet or appear to meet at a point on the principal axis.
2. This point at which the light rays converge or appear to converge is called the focus of the lens.
3. Now, a lens is made up of two spherical surfaces.
4. Hence, refraction can occur from both surfaces, resulting in two focal points.
5. Therefore, a lens has two foci.
6. The rays actually converge in the case of a convex lens.
7. The rays actually diverge and, upon tracing back, meet at the focus in the case of a concave lens.
8. The image formed on the focus is extremely diminished and inverted.
9. The distance between the focus and the pole is called the focal length.

Ray Diagram:

Therefore, a lens has two foci.