CUET · PHYSICS · PYQ PAPER 2025
A photo cathode is illuminated by a very small bright source placed 1 m away. When the same source of light is placed 0.5 m away, the number of electrons emitted by the photo cathode would become
- A four times
- B twice
- C one fourth
- D half
Answer & Solution
Correct Answer
(A) four times
Step-by-step Solution
Detailed explanation
\(N \propto I\) \(I \propto \frac{1}{r^2}\) \(N \propto \frac{1}{r^2}\) \(\frac{N_2}{N_1} = \left(\frac{r_1}{r_2}\right)^2\) \(\frac{N_2}{N_1} = \left(\frac{1}{0.5}\right)^2 = (2)^2 = 4\) four times
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