Radiation Pressure Calculator

Radiation pressure is the mechanical pressure exerted by electromagnetic radiation (light or other EM waves) upon a surface it strikes. It arises because photons carry momentum, and when they are absorbed or reflected, that momentum is transferred to the surface. While extremely small under everyday conditions, radiation pressure becomes significant inside stars and in space applications like solar sails.

For a surface with intensity I (W/m²) and reflection coefficient x (0 to 1), the radiation pressure is: P = I(1 + x) / c, where c is the speed of light (~3 × 10⁸ m/s). A perfectly absorbing surface (x = 0) experiences P = I/c, while a perfectly reflecting surface (x = 1) experiences P = 2I/c because momentum is transferred twice — once on absorption and once on reflection.

Solar sails exploit radiation pressure from sunlight to propel spacecraft without fuel. Large, lightweight, reflective membranes are deployed in space to capture photon momentum. Although the force is tiny (~9 μN/m² at Earth's distance from the Sun), it acts continuously and can accelerate a spacecraft significantly over time. Projects like IKAROS and LightSail 2 have demonstrated this technology.

The reflection coefficient (x) ranges from 0 to 1, where 0 means all radiation is absorbed and 1 means all radiation is perfectly reflected. A higher reflection coefficient results in greater radiation pressure because reflected photons impart twice the momentum of absorbed ones. Most real materials fall somewhere between these extremes.

At Earth's orbit, solar irradiance is about 1361 W/m² (the solar constant), producing a radiation pressure of roughly 4.5 × 10⁻⁶ Pa for a perfectly absorbing surface. This is negligible for everyday objects but must be accounted for in precise spacecraft navigation, satellite attitude control, and long-duration space missions.

Beyond solar sails, radiation pressure is used in optical tweezers to trap and manipulate microscopic particles and biological cells, in laser cooling to slow atoms to near absolute zero, and in precision metrology. High-powered laser systems can exert measurable forces on small particles, enabling contactless manipulation at the microscale.

In stellar interiors, temperatures are so extreme that the radiation field is enormously intense. Radiation pressure contributes significantly — and in massive stars, dominantly — to the total pressure that counterbalances gravitational collapse. It plays a crucial role in determining a star's structure, luminosity, and stability.

Radiation force is radiation pressure multiplied by the surface area: F = P × A = I(1 + x) × A / c. For example, with sunlight at 1361 W/m², a reflection coefficient of 1, and a sail area of 100 m², the force would be approximately 2 × 1361 × 100 / (3 × 10⁸) ≈ 9.07 × 10⁻⁴ N — about 0.9 millinewtons.