NEET Solved Paper 2014 Question 34
Question: Light with an energy flux of $ 25\times 10^{4}W{m^{-2}} $ falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 $ cm^{2} $ , the average force exerted on the surface is [AIPMT 2014]
Options:
A) $ 1.25\times {10^{-6}}N $
B) $ 2.50\times {10^{-6}}N $
C) $ 1.20\times {10^{-6}}N $
D) $ 3.0\times {10^{-6}}N $
Show Answer
Answer:
Correct Answer: A
Solution:
Energy flux = $ \frac{25\times 10^{4}J}{sm^{2}} $
Force on unit area = momentum transferred in unit time on unit area $ =\frac{25\times 10^{4}}{C} $
$ =\frac{25\times 10^{4}}{3\times 10^{8}} $ $ =,8.3\times {10^{-4}}N/m^{2} $
Force on the $ 15\times {10^{-4}}m^{2} $ area $ =8.3\times {10^{-4}}N/m^{2}\times 15\times {10^{-4}}m^{2} $
$ =124.5\times {10^{-8}}N $ $ 1.25\times {10^{-6}}N $
