JEE Main · 2005 · 08 May 2005 · Offline · Modern Physics

A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed

\frac{1}{2} m

away, the number of electrons emitted by photo cathode would

(A)decrease by a factor of 4
(B)increase by a factor of 4✓ Correct
(C)decrease by a factor of 2
(D)increase by a factor of 2

Answer: (B)

Solution

Light from a point source obeys the inverse-square law,

I \propto \frac{1}{r ^{2}}

, and the number of photoelectrons follows the intensity. Halving the distance from

1

m to

0.5

m multiplies the intensity by

(\frac{1}{0.5})^{2} = 4

, so the emitted electrons increase by a factor of

4

JEE Main · 2005 · 08 May 2005 · Offline · Single correct option

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