Table of Contents
What Is Radiation Pressure?
Radiation pressure is the mechanical pressure exerted on a surface by electromagnetic radiation (light, radio waves, etc.). Although photons have no rest mass, they carry momentum equal to their energy divided by the speed of light. When photons strike a surface, they transfer this momentum, creating a tiny but measurable force.
James Clerk Maxwell first predicted radiation pressure in 1862, and it was experimentally confirmed by Pyotr Lebedev in 1900. While extremely small under everyday conditions, radiation pressure has significant effects in astrophysics (stellar winds, comet tails) and is the driving principle behind solar sails for spacecraft propulsion.
Radiation Pressure Formulas
Radiation Pressure Examples
| Source | Intensity (W/m²) | Pressure (Pa) |
|---|---|---|
| Sunlight at Earth | 1,361 | 4.54 × 10-6 |
| Sunlight at Mercury | 9,100 | 3.03 × 10-5 |
| 1 mW Laser pointer | ~1,270 | 4.23 × 10-6 |
| High-power laser (1 kW/cm²) | 107 | 0.033 |
Applications
- Solar sails: Spacecraft propulsion using large reflective surfaces to harness sunlight momentum.
- Optical tweezers: Focused laser beams trap and manipulate microscopic particles using radiation pressure gradients.
- Stellar physics: Radiation pressure from a star's luminosity counteracts gravitational collapse, determining the star's equilibrium size.
- Laser ablation: Intense radiation pressure can ablate material surfaces for manufacturing and medical procedures.
Frequently Asked Questions
Why does a reflecting surface experience twice the pressure?
When light is absorbed, photons transfer their momentum p = E/c once. When light is reflected, the photon's momentum reverses direction, transferring 2p (initial momentum plus the recoil). This is analogous to a ball bouncing off a wall imparting twice the impulse compared to a ball that sticks.
Is radiation pressure significant in everyday life?
No. Solar radiation pressure at Earth's surface is about 4.5 micropascals, which is roughly 22 billion times weaker than atmospheric pressure. You would need a solar sail about 800 square meters in area to generate just 1 gram-force of thrust in space.
What is the Eddington luminosity?
The Eddington luminosity is the maximum luminosity a star can achieve before radiation pressure exceeds gravitational attraction, blowing away outer layers. It equals approximately 3.2 x 10^4 times (M/M_sun) solar luminosities. Stars exceeding this limit experience dramatic mass loss.