Thank you for reporting, we will resolve it shortly
Q.
An EM wave of intensity $I$ falls on a surface kept in vacuum and exerts radiation pressure $p$ on it. Which of the followings are true?
Electromagnetic Waves
Solution:
Radiation pressure $(p)$ is the force exerted by electromagnetic wave on unit area of the surface, i.e., rate of change of momentum per unit area of the surface.
Momentum per time per unit area $=\frac{\text { intensity }}{\text { speed of wave }}=\frac{I}{c}$
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 $(p)=\frac{I}{c}-\left(-\frac{I}{c}\right)=\frac{2 I}{c}$
Here, $p$ lies between $I / c$.