Radiation pressure is the pressure exerted by electromagnetic radiation (light, radio waves, X-rays, etc.) when it strikes a surface. This phenomenon occurs because electromagnetic waves carry momentum, and when they interact with matter, they transfer this momentum, creating a force. The concept was first predicted by James Clerk Maxwell in his electromagnetic theory and later experimentally confirmed by Lebedev, Nichols, and Hull in the early 20th century.
The Dual Nature of Light
Radiation pressure can be understood from both classical and quantum perspectives. In classical physics, electromagnetic waves carry energy and momentum, and when they strike a surface, they transfer both. In quantum physics, light consists of photons, each carrying a specific amount of momentum. When photons are absorbed or reflected, they transfer their momentum to the surface, creating pressure. This dual understanding helps explain why radiation pressure is fundamental to both wave and particle theories of light.
Maxwell's Prediction and Experimental Verification
James Clerk Maxwell predicted radiation pressure in 1873 as a consequence of his electromagnetic theory. He calculated that the pressure should be equal to the energy density of the electromagnetic field. The first experimental confirmation came in 1901 when Pyotr Lebedev measured the pressure of light on a small mirror suspended in a vacuum. Later, Ernest Nichols and Gordon Hull conducted more precise measurements, confirming Maxwell's predictions and establishing radiation pressure as a fundamental physical phenomenon.
The Momentum of Light
Electromagnetic radiation carries momentum despite having no rest mass. This momentum is given by p = E/c for photons, where E is the energy and c is the speed of light. When radiation strikes a surface, this momentum is transferred, creating a force. The magnitude of this force depends on whether the radiation is absorbed or reflected, with reflection transferring twice the momentum of absorption.