What this calculator does
Light carries momentum as well as energy, so it exerts a real pressure on whatever it strikes. The pressure is tiny, around 4.5 micropascals from full sunlight, but it is measurable and it accumulates usefully over large areas and long times.
A reflecting surface feels twice the pressure of an absorbing one, because the light reverses direction rather than simply stopping. That doubling is why solar sails are made reflective.
The formula
For an absorbing surface, divide the intensity by the speed of light. For a perfectly reflecting surface at normal incidence, double that, since the momentum change is twice as large when the light bounces back.
| Term | Meaning |
|---|---|
| Radiation pressure | Force per unit area exerted by electromagnetic radiation. |
| Solar constant | The roughly 1,361 W/m² of solar intensity at Earth's distance from the Sun. |
| Photon momentum | Energy divided by the speed of light. Photons have no mass but do carry momentum. |
The inputs explained
| Field | What to enter |
|---|---|
| Intensity (solar constant ≈ 1361) (W/m²) | The incident intensity in watts per square metre. Full sunlight at Earth's orbit is about 1,361. |
When to use it
Sizing a solar sail
Sail propulsion depends entirely on this pressure, so the required area follows from the thrust wanted.
Accounting for orbital perturbation
Solar radiation pressure gradually alters satellite orbits and has to be modelled for precise navigation.
Understanding optical tweezers
Focused laser light exerts enough pressure to trap and move microscopic particles, which is a Nobel-winning application.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much pressure does light exert?
A range of intensities from sunlight to concentrated laser light.
| Intensity | Pressure (absorbing) | Pressure (reflecting) |
|---|---|---|
| 1,361 W/m² | 4.5398e-6 Pa | 9.0796e-6 Pa |
| 10,000 W/m² | 3.3356e-5 Pa | 6.6713e-5 Pa |
| 100,000 W/m² | 3.3356e-4 Pa | 6.6713e-4 Pa |
Questions
How can light push if photons have no mass?
Because momentum does not require mass. A photon's momentum is its energy divided by the speed of light, and transferring that momentum to a surface is what produces the pressure.
Why does reflection double the pressure?
Because the momentum change is twice as large. An absorbed photon delivers its momentum and stops; a reflected one delivers that momentum and then takes the same amount again in the opposite direction.
Can a solar sail really work?
Yes, and it has been demonstrated in orbit. The pressure is minuscule, but it is continuous and needs no propellant, so over months it accumulates into a substantial velocity change.
Does this affect anything on Earth?
Negligibly in daily life, since atmospheric pressure is around 20 billion times larger. It matters in space, in precision measurements, and at the focus of powerful lasers.
For the energy the light carries, see the photon energy calculator. For total power radiated by a hot body, see the Stefan-Boltzmann law calculator.