Question

what is a focal length ?​

Answer 1

Answer:

Explanation:

Focal length of a lens is the distance between the focal point and the optical centre.

Answer 2

Answer:

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light.