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?
Answer & explanation
Correct answer: option 2
Momentum per unit time per unit area intensity = $\frac{\text { intensity }}{\text { speed of wave }}=\frac{I}{c}$
Change in momentum per unit time per unit area = ΔI/c = radiation pressure (P), i.e., P = ΔI/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 tie 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) = $\frac{I}{c}-\left(-\frac{I}{c}\right)=\frac{2 I}{c}$
Here, P lies between $\frac{I}{c}$ and $\frac{2 I}{c}$.