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Q.
An electromagnetic wave of intensity $I$ falls on a surface kept in vacuum and exerts radiation pressure $P$ on it. Which of the following statement is not true?
Electromagnetic Waves
Solution:
Momentum per unit time per unit area
$= \frac{intensity}{speed\, of \,wave} = \frac{I}{c}$
Change in momentum per unit time per unit area $= \Delta I/c =$ radiation pressure $\left(P\right)$, i.e. $P =\Delta I/c$.
Momentum of incident wave per unit time per unit area $-I/c$
When wave is fully absorbed by the surface, the momentum of the reflected wave per unit time per unit area $= 0$
Radiation pressure $\left(P\right) = $ change in momentum per unit time per unit area $= \frac{\Delta I}{c} = \frac{I}{c}-0 = \frac{I}{c}$
When wave is totally reflected, then momentum of the reflected wave per unit time per unit area $= -I/c$.
Radiation pressure $\left(P\right) = \frac{I}{c}-\left(-\frac{I}{c}\right) = \frac{2I}{c}$
Here, $P$ lies between $\frac{I}{c}$ and $\frac{2I}{c}$.