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  • What is the force exerted by light beam if light is incident on surface at an angle as shown in figure

What is the force exerted by light beam if light is incident on surface at an angle \theta as shown in figure if on the rays are completely reflected.

 

Option: 1

\frac{I \cos ^2 \theta}{c}


Option: 2

\frac{2 I \cos ^2 \theta}{c}


Option: 3

\frac{I}{C}


Option: 4

\frac{I \cos \theta}{c}


Answers (1)

best_answer

Change in momentum of one photon
=\frac{2 h}{\lambda} \cos \theta \text { (upward) }
Number of photons incident per unit time 

=\frac{\text { Energy indent per unit time }}{h \nu}
=\frac{I A \cos \theta}{h \nu}=\frac{I A \lambda \cos \theta}{h c}
\Rightarrow Total change in momentum per unit time =\frac{I A \cos \theta \cdot \lambda}{h c} \cdot \frac{2 h}{\lambda} \cos \theta=\frac{2 I A \cos ^2 \theta}{C} (upward)
\Rightarrow  Force on the plate =\frac{2 I A \cos ^2 \theta}{C} \text { (downword) } \\
\Rightarrow pressure =\frac{2 I A \cos ^2 \theta}{A \cdot C}=\frac{2 I \cos ^2 \theta}{C}.

Posted by

Kuldeep Maurya

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