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Question

When a monochromatic point source of light is at a distance 0.2 m from a photoelectric cell, the saturation current and cut-off voltage are 18.0mA and 0.6V. If the same source is placed 0.6m away from the photoelectric cell, then the saturation current and the stopping potential respectively are :

A
4 mA and 1V
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B
2mA and 0.6V
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C
3 mA and 0.5V
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D
12 mA and 0.5V
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Solution

The correct option is D 2mA and 0.6V
The voltage and current depend on number of photons coming to the photo electrical cell. Now, number of photon will vary as 1r2

So, Voltage will vary like this and we can write

V1V2=r22r21

then V2=r21×V1r22

V2=0.22×0.60.62

V2=0.69=0.06V

I1I2=V1V2 then I2V2×I1V1=0.06×180.6=2mA

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