Consider normal incidence of a monochromatic beam of photons of power P on a flat surface. Of the incident beam, 10% gets absorbed, 10% gets transmitted, and the rest is reflected by the flat surface. If c is the speed of light, what is the force exerted on the flat surface by the beam?

Step-by-step solution

The total incident momentum of the photons per second is given by p_i = P c . The incident beam is divided into three parts: absorbed, reflected, and transmitted. 1. Absorbed part: 10 % of the beam is absorbed. The photons lose all their momentum. The force exerted by this part is the rate of change of momentum: F_ absorbed = 0.1 P c 2. Reflected part: 80 % of the beam is reflected. The photons reverse their direction, so the change in momentum is twice their initial momentum. The force exerted by this part is: F_