Radiation Pressure Calculator

Calculate the pressure exerted by light and the optional force on a surface.

For absorbed light, P = I/c. For perfectly reflected light, P = 2I/c.

About the Author: Created by Fotios Angelakis, MSc in Mechanical Engineering, with experience in thermodynamics, energy systems, and engineering calculations. Learn more about the author's qualifications and experience.

Enter intensity and click calculate.

How the Radiation Pressure Calculator Works

The Radiation Pressure Calculator estimates the pressure exerted by electromagnetic radiation on a surface. Light carries momentum, so when it is absorbed or reflected, it transfers momentum to the surface and creates a small pressure.

Radiation pressure is usually tiny in everyday conditions, but it becomes important for solar sails, lasers, astrophysics, and precision optical systems.

Incoming light intensity I pressure P P = I/c or P = 2I/c

Radiation Pressure Formulas

For a surface that absorbs light:

P = I / c

For a perfectly reflecting surface:

P = 2I / c

where I is intensity in W/m² and c is the speed of light.

Force from Radiation Pressure

If the surface area is known, the total force is:

F = P · A

where F is force in newtons, P is pressure in pascals, and A is area in square meters.

How to Use the Calculator

  1. Enter the light intensity or select a predefined source.
  2. Choose whether the light is absorbed or perfectly reflected.
  3. Optionally enter a surface area.
  4. Click Calculate Radiation Pressure.
  5. Review pressure in Pa, μPa, and optional force in N.

Example: Sunlight

For sunlight with intensity about 1000 W/m²: 

I = 1000 W/m²
c = 299,792,458 m/s

Absorbed pressure:

P = 1000 / 299792458 ≈ 3.34 × 10⁻⁶ Pa

Reflected pressure:

P = 2 × 1000 / 299792458 ≈ 6.67 × 10⁻⁶ Pa

Example: High-Intensity Laser

For a laser intensity of 1,000,000 W/m²: 

I = 1,000,000 W/m²

Absorbed pressure:

P ≈ 3.34 × 10⁻³ Pa

Reflected pressure:

P ≈ 6.67 × 10⁻³ Pa

Applications of Radiation Pressure

  • Solar sails: using sunlight momentum for spacecraft propulsion.
  • Laser systems: estimating forces from intense laser beams.
  • Astrophysics: modeling dust movement, stellar winds, and star formation.
  • Optical instruments: accounting for tiny forces in precision systems.
  • Laboratory physics: understanding photon momentum transfer.
Important: This calculator assumes normal incidence and either full absorption or perfect reflection. Real surfaces may partially absorb, reflect, and scatter light.

Frequently Asked Questions

Why does light create pressure?

Light carries momentum. When that momentum is absorbed or reflected by a surface, it produces force over an area.

Why is reflected pressure twice the absorbed pressure?

A perfectly reflected photon reverses direction, so the momentum change is about twice as large as for absorption.

Is radiation pressure large?

Usually it is very small, but it becomes important over large areas, long times, or very high intensities.

Does angle matter?

Yes. This calculator assumes the light strikes the surface directly. Angled light changes the effective pressure.

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