Radiation Pressure
P = I/c (absorbed) or 2I/c (reflected). Light pushes objects.
Key Notes
Electromagnetic waves carry momentum p = E/c (per photon, p = h/λ). When they hit a surface, they exert a pressure called radiation pressure.
Perfect absorber: momentum E/c is fully transferred → pressure P_abs = I/c.
Perfect reflector: photon reverses direction, momentum change is 2E/c → pressure P_ref = 2I/c.
Partial absorber (reflectance R, 0 ≤ R ≤ 1): P = (1+R)·I/c.
Effect is tiny in everyday life — solar radiation gives ~5 μPa on absorbing surfaces and ~10 μPa on mirrors — but real and measurable.
Applications: solar sails (NEA Scout, IKAROS spacecraft) propel themselves without fuel; optical tweezers manipulate cells using laser radiation pressure; comet tails are pushed away from the Sun by both radiation pressure and solar wind.
Astrophysical importance: in massive stars, radiation pressure can dominate gravity (Eddington limit) — sets the maximum stable luminosity of a star.
Formulas
Pressure on a perfect absorber
I = intensity (W/m²); c = speed of light.
Pressure on a perfect reflector
Double — because the photon's momentum reverses.
General surface (reflectance R)
0 ≤ R ≤ 1: 0 = absorber, 1 = mirror.
Force on area A
Useful for solar-sail / pendulum-experiment problems.
Eddington luminosity
Max luminosity at which radiation force balances gravity for a hydrogen plasma.
Important Points
Direction of force on a sail is ALONG the propagation direction of the light, NOT necessarily Sun→ship — it depends on the sail's orientation (reflected component matters).
Comet tails: gas tail (Type I) is pushed by solar WIND (ions); dust tail (Type II) is pushed by RADIATION PRESSURE. Both point away from the Sun.
For most engineering problems, mirror pressure is preferable (2×) → solar sails use aluminised Mylar.
Common mistake: forgetting the factor of 2 for a mirror. Always identify if the surface absorbs or reflects.
Radiation pressure ≠ radiation FORCE. Pressure is intensive (Pa); force depends on area.
On Earth's surface, atmospheric pressure (≈10⁵ Pa) is ~10¹¹ × larger than solar radiation pressure — that's why we don't feel it.
Radiation Pressure notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.