Lens-Principal Focus

Description:

Principle focus number 1 of concave lens is that point on the principle axis, such that all incoming rays directed towards it become parallel to principle axis after refraction.

Principle focus 2 of a concave lens is that point on principle axis from where, all parallel incoming ray appear to come.

Principle focus number 1 of convex lens is that point on the principle axis, from which all incoming rays, after refraction become parallel.

Principle focus 2 of a convex lens is that point on principle axis where, all parallel incoming ray intersect after refraction.

The distance from optical center of lens to focus is known as focal length. Focal length is same for principle focus 1 and principle focus 2.

Principle focus for convex and concave lens are opposite in direction.

Focal length of convex lens is positive and for concave lens it is negative.