Page 31 - cbse-class-10-light-reflection-and-refraction

It is a point on the lens's principal axis at which light rays parallel to the lens's
principal axis after refraction from both surfaces of the lens pass through (convex
lens) or appear to come from this point (concave lens).

The distance between the optic center and the second principal focus is referred to
as the lens's second focal length (f ). The first and second focal lengths will be
2
equal if the medium on both sides of the lens is the same. The focus of a convex
lens is physical, whereas the focus of a concave lens is virtual.

Sign Convention for Spherical Lenses

All distances are measured from the lens's optical center. Distances measured in
the direction of the incident light are considered positive, while distances measured
in the opposite direction of the incident light are considered negative. All
measurements taken above the principal axis are considered positive, while
measurements taken below the principal axis are considered negative, i.e., object
height is always considered positive, while image height is only considered
positive for virtual images.

Class X Science www.vedantu.com 31