Question

what is focal length...​

Answer 1

Answer:

distance between the center of a convex lens or a concave mirror and the focalpoint of the lens or mirror — the point where parallel rays of light meet, or converge...

Answer 2

$\huge{ \boxed{\bf\red{ANSWER}}}$

□ It is the half of the distance from the centre of Curvature.

□ f = R/2