Question

define focal length and lens ​

Answer 1

Answer:

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive, and is the distance at which a beam of collimated light will be focused to a single spot.

Answer 2

Answer:

distance btwin principal focus and optical centre

Explanation:

is called focal length

in simple words

don't go for complicated words