A) carry one quarter of their previous energy
B) carry one quarter of their previous momentum
C) are half numerous
D) are one quarter numerous
Correct Answer: D
Solution :
When light falls normally on a surface, the illuminance is maximum. If from a light source of luminous intensity\[(I),\]light is falling normally on a surface distant r from the source, then illuminance is \[E=\frac{I}{{{r}^{2}}}\] Given, \[{{r}_{1}}=2m,{{r}_{2}}=4m\] \[\therefore \] \[\frac{{{E}_{1}}}{{{E}_{2}}}=\frac{r_{2}^{2}}{r_{1}^{2}}\] \[\frac{{{E}_{1}}}{{{E}_{2}}}=\frac{{{(4)}^{2}}}{{{(2)}^{2}}}=4\Rightarrow {{E}_{2}}=\frac{{{E}_{1}}}{4}\] Hence, electrons emitted by photo cathode in one second are one quarter numerous.
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