Em Waves Question 75
Question: How many photons of a radiation of wavelength $ \lambda =5\times {{10}^{-7}}m $ must fall per second on a blackened plate in order to produce a force of $ 6.62\times {{10}^{-5}} $ N?
Options:
A) $ 3\times 10^{19} $
B) $ 5\times 10^{22} $
C) $ 2\times 10^{22} $
D) $ 1.67\times 10^{18} $
Show Answer
Answer:
Correct Answer: A
Solution:
[a] Let the number of photons hitting the photocell per second be n, then $ nhv=1.5mW $ Or $ n=\frac{(1.5\times {{10}^{-3}})\times 400\times {{10}^{-9}}}{(6.63\times {{10}^{-34}})\times (3\times 10^{8})} $
$ [ \
Therefore v=\frac{c}{\lambda }=\frac{3\times 10^{8}}{400\times {{10}^{-9}}} ] $
$ \
Therefore n=3\times 10^{15} $ Number of photoelectrons produced per second is $ 3\times 10^{15}\times \frac{0.1}{100}=(3\times 10^{12}) $ So, current in the photocell is $ (3\times 10^{12})(1.6\times {{10}^{-19}})=4.8\times {{10}^{-7}}A $
BETA
