Dual Nature of Radiation and Matter - Result Question 51
54. A photoelectric cell is illuminated by a point source of light $1 m$ away. When the source is shifted to $2 m$ then
[2003]
(a) number of electrons emitted is a quarter of the initial number
(b) each emitted electron carries one quarter of the initial energy
(c) number of electrons emitted is half the initial number
(d) each emitted electron carries half the initial energy
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Answer:
Correct Answer: 54. (a)
Solution:
- (a) Intensity $\propto$ No. of electrons emitted (N)
$I \propto \frac{1}{r^{2}} \Rightarrow N \propto \frac{1}{r^{2}}$
Energy crossing per unit area normally per second is called Intensity.
$I=\frac{E}{A t}=\frac{P}{A} \quad(.$ Here, $P=\frac{E}{t}=$ radiation power $)$
At a distance $r$ from a point source of power $P$
intensity is given by $I=\frac{P}{4 \pi r^{2}}$
$\Rightarrow I \propto \frac{1}{r^{2}}$
