Electromagnetic Waves - Result Question 6

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6. Light with an energy flux of $25 \times 10^{4} Wm^{-2}$ 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 :[2014]

======= ####6. Light with an energy flux of $25 \times 10^{4} Wm^{-2}$ 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 :[2014]

3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed:content/english/neet-pyq-chapterwise/physics/electromagnetic-waves/electromagnetic-waves—result-question-6.md (a) $1.25 \times 10^{-6} N$

(c) $1.20 \times 10^{-6} N$

(b) $2.50 \times 10^{-6} N$

(d) $3.0 \times 10^{-6} N$

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Answer:

Correct Answer: 6. (b)

Solution:

  1. (b) Average force $F _{av}=\frac{\Delta p}{\Delta t}=\frac{2 IA}{c}$

$ \begin{aligned} & =\frac{2 \times 25 \times 10^{4} \times 15 \times 10^{-4}}{3 \times 10^{8}} \\ & =2.50 \times 10^{-6} N \end{aligned} $

Pressure $=\frac{\text{ Force }}{\text{ Area }}=\frac{F}{A}$

Force is the rate of change of momentum $F=\frac{d p}{d t}$

But energy, $U=p c$

$ \therefore \text{ Pressure }=\frac{1}{A} \frac{U}{c \cdot d t}=\frac{I}{c} $

$ [\because \text{ Intensity }=\frac{U}{A d t}] $

For perfectly reflecting surface, $P_r=\frac{2 I}{c}$

$\therefore$ For perfectly reflecting surface, Force $=\frac{2 I A}{c}$