Question

Which of the following can be used to find focal length of a lens?

(a) Light from sun (b) Light from a distant tree
(c) Light from window of our lab. d) Object at a distance of 0 cm for a focal length of 30 cm.

For XIII class CBSE.

Answer 1

most accurate focal lenght will be obtained if you use light from Sun.
In Practice, Light from a distant tree can also be used.

Light from window may be used if it is away by more than 10 times the focal length, that is, 3 meters.

d) it is not possible to determine focal length with a distance of 0cm