Light carries momentum, so it pushes on whatever it hits. Near Earth the push is tiny – about 9 µN per square metre for a mirror – but solar sails like IKAROS and LightSail have used it for propulsion. Inside massive stars radiation pressure helps hold them up.
How to use the Radiation Pressure Calculator
- Location
- Surface
- Angle of incidence θ
- Star luminosity (L☉)
- Distance from the star
- Temperature inside the star
- Sail area (optional)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose outside the star (surface in sunlight) or inside the star.
Inputs explained
| Field | What to enter |
|---|---|
| Location | Choose one: Outside the star, Inside the star |
| Surface | Choose one: Perfectly absorbing (opaque), Perfectly reflecting |
| Angle of incidence θ | Units: °, rad, arcmin, arcsec, mrad |
| Star luminosity (L☉) | A number |
| Distance from the star | Units: au, km, R☉ |
| Temperature inside the star | Units: K, °C, °F |
| Sail area (optional) | Units: m², cm², mm², km², in², ft² |
What each result means
- Intensity at that distance = 3.828e26×[Star luminosity (L☉)]÷(4×pi×[Distance from the star]^2)
- Force on the sail = [Radiation pressure]×[Sail area (optional)]
Formula
Outside: P = (L / (4πd²c))·(1 + R)·cos²θ (R = 0 absorbing, 1 reflecting) · Inside: P = aT⁴/3, a = 7.566 × 10⁻¹⁶ J/(m³·K⁴)
Worked example
Inputs — Location: Outside the star; Surface: Perfectly absorbing (opaque); Angle of incidence θ: 0 °; Star luminosity (L☉): 1; Distance from the star: 1 au; Sail area (optional): 100 m².
Results — Intensity at that distance = 1,361.16647 W/m²; Radiation pressure = 4.54036 µPa; Force on the sail = 454.03626 µN.
Key terms
- Solar sail
- A large reflective sheet propelled by light.
Frequently asked questions
How strong is sunlight's pressure at Earth?
About 4.6 µPa absorbed, 9.1 µPa reflected.
Why does a mirror get twice the push?
Reflected light reverses its momentum.
Can radiation pressure move spacecraft?
Yes – solar sails use it, and it perturbs satellite orbits.
Why is it huge inside stars?
It grows with T⁴ and stellar cores are millions of kelvins.