Radiation Pressure Calculator
The pressure and force exerted by light on a surface — absorbing or reflecting — and what it means for a solar sail.
How hard sunlight pushes.
How the radiation pressure calculator works
Light carries momentum p = E/c, so a beam striking a surface exerts pressure. For a fully absorbing surface P = I/c; for a perfect mirror it doubles to 2I/c, because the light reverses direction.
The numbers are tiny — full sunlight at Earth's distance exerts about 4.6 micropascals on a black surface, roughly the weight of a grain of sand spread over a square meter. Over months of continuous thrust, that is still enough to move a spacecraft.
Formula: P = I/c (absorbed); P = 2I/c (reflected)
Worked examples
| Inputs | Radiation pressure | Note |
|---|---|---|
| A 1,000 m² solar sail at Earth | 0.00000908 Pa | 9.1 µPa, 9.1 mN, 2.9 km/s a year |
| A black surface instead | 0.00000454 Pa | half the force |
| Close to the Sun | 0.0000908 Pa | ten times the intensity, ten times the push |
FAQFrequently asked questions
Does light really push things?
Yes. Photons carry momentum, and transferring it exerts a real, measurable force — first demonstrated by Lebedev in 1899.
How strong is sunlight's push?
About 9 µPa on a mirror at Earth's distance. Over a square meter that is 9 micronewtons — the weight of a milligram.
Why does a mirror get twice the push?
Because reflecting reverses the photon's momentum rather than merely absorbing it, so twice the momentum is transferred.
Do solar sails actually work?
Yes. IKAROS reached Venus in 2010 on sunlight alone, and LightSail 2 raized its orbit in 2019.
Why is a sail worth it if the force is so small?
Because it never stops and needs no propellant. Milligrams of thrust applied for years builds up speeds chemical rockets cannot match.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.