Practicing Success
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? |
Radiation pressure is I/c if the wave is totally absorbed Radiation pressure is I/c if the wave is totally reflected Radiation pressure is 2 I/c if the wave is totally reflected Radiation pressure is in the range I/c < p > 2 I/c for real surfaces |
Radiation pressure is I/c if the wave is totally reflected |
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 $\frac{2 I}{c}$. |