• Tags: intensity, pressure, force
• Difficulty: Advanced

Intensity I = Power/Area = (1/2)cepsilon_0E_0^2 = E_0^2/(2mu_0c) = cB_0^2/(2mu_0). For a point source of power P at distance r: I = P/(4pi*r^2) (inverse square law). Radiation pressure on a surface: P_rad = I/c (perfect absorber) or 2I/c (perfect reflector) or (1+r)I/c for partial reflection (reflectivity r). Force F = P_rad x A. Momentum delivered per second = F = Power absorbed/c or 2Power reflected/c. Example: sunlight intensity at Earth ≈ 1400 W/m^2. Radiation pressure on a black surface = 1400/(3x10^8) ≈ 4.7 x 10^-6 Pa — negligible compared to atmospheric pressure but detectable with sensitive instruments. For a perfectly reflecting surface, the pressure doubles to ~9.4 x 10^-6 Pa. Radiation pressure is responsible for comet tails pointing away from the Sun.