A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s.
A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is
(a) 3 s
(b) 12 s
(c) 24 s
(d) 48 s.
4 views