Question

What is (second) focal length?? ​

Answer 1

Answer:

Second Focal plane:A vertical line passing through second principal focus. The distance between optical centre and second principal focus is called second principal focal length.

Answer 2

$\mathfrak{\huge\underline{Answer:}}$

The distance between the convex lens or a concave mirror and the focal point of a lens or mirror is called the focal length. It is the point where parallel rays of light meet or converge. The focal length varies with the sign (positive or negative) depending upon the nature of the lens and mirror(concave or convex).