Radiation Pressure from Light
Light carries momentum, so it exerts a tiny but real pressure on any surface it strikes. For a fully absorbing surface, radiation pressure is P = I/c, where I is the light’s intensity (power per unit area) and c is the speed of light. For a fully reflecting surface, the pressure doubles to P = 2I/c, because the light’s momentum is reversed rather than simply absorbed.
Using the solar constant (about 1361 W/m² at Earth’s distance from the Sun) as intensity gives the radiation pressure from sunlight, which is the basis for solar sail propulsion concepts. Because c is so large, radiation pressure from ordinary light sources is extremely small, but it becomes significant over large areas, long timescales, or extremely high-intensity sources like lasers.