Radiation Pressure Calculator.

Calculate pressure from intensity and surface reflectivity.

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Intensity

Reflectivity (0–1)

Radiation pressure

0.000000Pa

Results update automatically as you type.

Use Cases

Physics education and experiments

Understand the momentum of light and verify theoretical predictions in lab setups. Use it to compute expected pressure for laser beams or light sources in classroom demonstrations.

Example: A 5 mW laser focused to 1 mm² gives intensity 5000 W/m². With R=0, pressure ≈ 1.67×10⁻⁵ Pa.

Frequently Asked Questions

How is radiation pressure calculated?: Radiation pressure is calculated using the formula P = I/c * (1 + R) for normal incidence, where I is intensity in W/m², c is the speed of light (about 3×10⁸ m/s), and R is reflectivity (0–1). For a perfect absorber (R=0), pressure is I/c; for a perfect reflector (R=1), it's 2I/c.
What does reflectivity mean in this calculator?: Reflectivity (0–1) is the fraction of incident light reflected by the surface. A value of 0 means the surface absorbs all light, while 1 means it reflects all light. Higher reflectivity increases radiation pressure because reflected light transfers more momentum.
What units does the result use?: The result is in pascals (Pa), the SI unit of pressure. For typical light intensities, the pressure is very small, often in micropascals (µPa) or nanpascals (nPa).

Tips & Common Mistakes

Tips

Ensure intensity is in watts per square meter (W/m²). If you have power in watts and area in m², divide power by area to get intensity.
Reflectivity must be between 0 and 1. Use 0 for a perfectly absorbing surface (black body) and 1 for a perfectly reflecting mirror.
For oblique incidence, the pressure is lower; this calculator assumes normal (perpendicular) incidence.
Remember that radiation pressure is extremely small for everyday light sources; it becomes significant only for intense beams or in space applications.

Common Mistakes to Avoid

Using reflectivity as a percentage (e.g., 90 instead of 0.9). Always convert percentages to a decimal between 0 and 1.
Forgetting to convert intensity to W/m². If you have power in milliwatts or area in cm², convert to base SI units first.
Assuming the result is in atmospheres or another pressure unit; the calculator outputs pascals (Pa).

Last updated: August 13, 2026

Use Cases

Physics education and experiments

Understand the momentum of light and verify theoretical predictions in lab setups. Use it to compute expected pressure for laser beams or light sources in classroom demonstrations.

Example: A 5 mW laser focused to 1 mm² gives intensity 5000 W/m². With R=0, pressure ≈ 1.67×10⁻⁵ Pa.

Frequently Asked Questions

How is radiation pressure calculated?

Radiation pressure is calculated using the formula P = I/c * (1 + R) for normal incidence, where I is intensity in W/m², c is the speed of light (about 3×10⁸ m/s), and R is reflectivity (0–1). For a perfect absorber (R=0), pressure is I/c; for a perfect reflector (R=1), it's 2I/c.

What does reflectivity mean in this calculator?

Reflectivity (0–1) is the fraction of incident light reflected by the surface. A value of 0 means the surface absorbs all light, while 1 means it reflects all light. Higher reflectivity increases radiation pressure because reflected light transfers more momentum.

What units does the result use?

The result is in pascals (Pa), the SI unit of pressure. For typical light intensities, the pressure is very small, often in micropascals (µPa) or nanpascals (nPa).

Tips & Common Mistakes

Tips

Ensure intensity is in watts per square meter (W/m²). If you have power in watts and area in m², divide power by area to get intensity.
Reflectivity must be between 0 and 1. Use 0 for a perfectly absorbing surface (black body) and 1 for a perfectly reflecting mirror.
For oblique incidence, the pressure is lower; this calculator assumes normal (perpendicular) incidence.
Remember that radiation pressure is extremely small for everyday light sources; it becomes significant only for intense beams or in space applications.

Common Mistakes to Avoid

Using reflectivity as a percentage (e.g., 90 instead of 0.9). Always convert percentages to a decimal between 0 and 1.
Forgetting to convert intensity to W/m². If you have power in milliwatts or area in cm², convert to base SI units first.
Assuming the result is in atmospheres or another pressure unit; the calculator outputs pascals (Pa).

Use Cases

Physics education and experiments

Understand the momentum of light and verify theoretical predictions in lab setups. Use it to compute expected pressure for laser beams or light sources in classroom demonstrations.

Example: A 5 mW laser focused to 1 mm² gives intensity 5000 W/m². With R=0, pressure ≈ 1.67×10⁻⁵ Pa.

Frequently Asked Questions

How is radiation pressure calculated?

What does reflectivity mean in this calculator?

What units does the result use?

The result is in pascals (Pa), the SI unit of pressure. For typical light intensities, the pressure is very small, often in micropascals (µPa) or nanpascals (nPa).

Tips & Common Mistakes

Tips

Ensure intensity is in watts per square meter (W/m²). If you have power in watts and area in m², divide power by area to get intensity.
Reflectivity must be between 0 and 1. Use 0 for a perfectly absorbing surface (black body) and 1 for a perfectly reflecting mirror.
For oblique incidence, the pressure is lower; this calculator assumes normal (perpendicular) incidence.
Remember that radiation pressure is extremely small for everyday light sources; it becomes significant only for intense beams or in space applications.

Common Mistakes to Avoid

Using reflectivity as a percentage (e.g., 90 instead of 0.9). Always convert percentages to a decimal between 0 and 1.
Forgetting to convert intensity to W/m². If you have power in milliwatts or area in cm², convert to base SI units first.
Assuming the result is in atmospheres or another pressure unit; the calculator outputs pascals (Pa).