Question

A photographic plate records sufficiently intense lines when it is exposed for 12 s to a source of 10 W. How long should it be exposed to a 12 W source radiating the light of same colour to get equally intense lines?

Answer 1

The time needed is 10 seconds.

Source of power = 10W (Given)

Time = 12s (Given)

Let the time for which the photograph gets exposed be = t

For recording the intense lines, the energy should be the same. \

Thus,

Energy = radiant flux × time

Energy = 10W × 12s

Energy = 12W × t

t = 10w × 12s / 12w

t = 10s

Thus, the time needed is 10 seconds.

Answer 2

10s, the photographic plate should be exposed to a 12 W source radiating the light of same colour to get equally intense lines.

Explanation:

Let t be the time for the photographic plate is being exposed for.

Power should be the same for capturing the lines that are strong enough.

Given data in the question

Time (t) = 12 s

Radiant flux = 10 W to 12 W

Energy = radiant flux × time

           $=10 \mathrm{W} \times 12 \mathrm{s}$

           $=12 W \times t$

         $t=\frac{(10 W \times 12 s)}{12 W}=10 s$

           = 10 s

Hence, to get equally intense images, the photographic plate should be exposed for almost 10 s.