Question

Define Focal length I am fine
How are you? ​

Answer 1

Answer:

the distance between the centre of the mirror or a lens and its focus

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.