Question

what is mean by focal length?​

Answer 1

Explanation:

FOCAL LENGTH : -

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

एक दर्पण या लेंस के केंद्रबिंदु से उसके फ़ोकस की दूरी; नाभीय दूरी

Answer 2

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.

Formula

$\huge f=-\frac{R}{2}$

f = focal length

R = radius of curvature of the mirror's surface