Question

An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?

1. Radiation pressure is `I/c` if the wave is totally absorbed.
2. Radiation pressure is `I/c` if the wave is totally reflected.
3. Radiation pressure is `(2I)/c` if the wave is totally reflected.
4. Radiation pressure is in the range `I/c < p < (2I)/c` for real surfaces.

Options

• a, b and c

• b, c and d

• a, c and d

• c and d

Answer 1

a, c and d

Explanation:

Radiation pressure (p) is the force exerted by electromagnetic waves on unit area of the surface, i.e., rate of change of momentum per unit area of the surface.

Let us consider a surface exposed to electromagnetic radiation as shown in figure. The radiation is falling normally on the surface. Further, intensity of radiation is I and area of surface exposed to radiation is A.

Radiation pressure is the force exerted by particles (dual nature of particle) on unit area, due to the charge in momentum of radiated particles per unit area per sec = `I/c`.

I = Intensity pf radiation

C = Velocity of radiation

Radiations are absorbed, so momentum per unit area second = `I/c`.

When radiation is reflected back, the momentum becomes double as in earlier case.

So variation of radiation pressure p comes between the range `I/c < p < (2I)/c`.