Question

An electromagnetic wave of intensity falls on a surface kept in vacuum and exerts radiation pressure P on it. Which of the following statement is not true?

• A. Radiation pressure is I/c if the wave is totally absorbed.
• B. Radiation pressure is I/c if the wave is totally reflected
• C. Radiation pressure is 2 I/c if the wave is totally reflected.
• D. Radiation pressure is in the range I/c < p>2 I/c for real surfaces.

Answer 1

Answer: (B)

Radiation pressure is I/c if the wave is totally reflected

Answer 2

: Momentum per unit time per unit area

$= \frac{{int}ensity}{speedofwave} = \frac{I}{c}$

Change in momentum per unit time per unit area $= \Delta I/c =$ radiation pressure (P), 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 (P) = 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 $(P) = \frac{I}{c} - \left( - \frac{I}{c} \right) = \frac{2I}{c}$

Here, P lies between $\frac{I}{c}$and $\frac{2I}{c}$