Photon Momentum
p = h/λ — Light carries momentum even with zero rest mass
Photon–Mirror Collision
Wavelength λ:
500
nm
p = h/λ = 6.626×10⁻³⁴ / λ
Δp (mirror) = 2p (elastic reflection)
Impulse = 2h/λ ≈
2.65
× 10⁻²⁷ kg·m/s
Each photon transfers twice its momentum to the mirror upon reflection. This is the origin of radiation pressure.
Solar Sail: Thrust vs Distance
Sail area:
1000
m²
Distance from Sun:
1.0
AU
Radiation pressure at 1 AU: 4.56 μPa
Thrust:
4.56
mN | Acceleration:
--
mm/s²
Solar sails use photon momentum. Thrust ∝ 1/d². At 10 AU (Saturn), thrust drops 100×.