A 5 watt source emits monochromatic light of wavelength 5000 ; A.… - Vidyadip

MCQ

A $5$ watt source emits monochromatic light of wavelength $5000\; \mathring A$. When placed $0.5\; m$ away, it liberates photoelectrons from a photosensitive metallic surface. When the source is moved to a distance of $1.0\;m$, the number of photo electrons liberated will

✓
be reduced by a factor of $4$
B
be reduced by a factor of $2$
C
be reduced by a factor of $8$
D
be reduced by a factor of $16$

✓

Answer

Correct option: A.

be reduced by a factor of $4$

a
Power of the source $P_0=5\; W$

Power at the distance $r$ from source, $P=\frac{P_0}{4 \pi r^2}$

Each photon emits one electrons.

Also, $P \propto N$

$N$ is the number of photons

$N \propto \frac{1}{r^2}$ $\Rightarrow \frac{N_2}{N_1}=\frac{r_1^2}{r_2^2}$

$\frac{N_2}{N_1}=\frac{0.5^2}{1^2}=\frac{1}{4}$

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