Question

White light is incident on three identical surfaces - a black surface, a yellow surface and a white surface, one by one. For which surface, the pressure exerted on the surface by the incident light will be:

1. maximum
2. minimum

Justify your answer.

Answer 1

When light falls on a surface, it exerts radiation pressure due to momentum transfer. The kind of surface determines the pressure:

Black Surface: A black body absorbs all the incident light, so the momentum transferred is:

ρ = `E/c`

White Surface: A white body reflects all the light, so the total change in momentum is 2ρ, leading to maximum pressure.

Yellow Surface: A partially reflective surface (like yellow) absorbs some light and reflects the rest, so the pressure is intermediate.

1. Maximum pressure: White surface (due to full reflection)
2. Minimum pressure: Black surface (due to full absorption)