Question

What is focal lenght?

Answer 1

The explanation is as follows.

Going by the definition - focal length is the distance between the centre of a lens or curved mirror and its focus.

In other words it if the distance from the centre of lense or mirror where the parallel beam of light coming from infinite converges.

It is half of the radius of the sphere of which the lense or mirror is part of.

Please try to visualise the above statements to understand better.

Hope it helps.
All the best!