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

Intensity I = $\frac{Power}{Area}$ = $\frac{1}{2}$c$epsilon_0$$E_0^2$ = $E_0$^$\frac{2}{2*mu_0*c}$ = c$B_0$^$\frac{2}{2*mu_0}$. For a point source of power P at distance r: I = $\frac{P}{4*pi*r^2}$ (inverse square law). Radiation pressure on a surface: $P_{rad}$ = $\frac{I}{c}$ (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 = $\frac{1400}{3x10^8}$ ≈ 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.