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Reflection from Metals

Let's begin simply by considering reflection from the surface of an ideal metal (ie, a perfect mirror). When we say we have an ideal metal, we actually think of it as perfect conductor. A metal contains many free or nearly-free electrons. In a perfect conductor these electrons can move freely under the influence of an electric field. As a consequence a perfect conductor is field free. If there were an electric field the electrons would start to move and the displacement of charges will just compensate the electrical field. As a consequence there is none.

Let us consider the incoming photons as an electromagnetic wave. The experiment tells us that the light or electromagnetic wave is being reflected at the metal surface. The assumption that we have a perfect conductor implis now two different boundary conditions. First the component of the electrical field parallel to the metal surface must be zero. Secondly the component of the magnetic field perpendicular to the metal surface must also vanish.

 

A photon travels at the speed of light in a definite direction, carrying momentum. When it reaches the surface of the metal, the collision must involve the conservation of energy and momentum.

These electrons dominate the electromagnetic (EM) field in the metal and form an EM barrier at the surface. The incoming photon is reflected by this barrier.

Conservation of momentum requires that the reflection occurs in the plane of the incoming photon and that the incident angle equals the exit angle. The conservation of energy means the wavelength of the photon does not change on reflection.

First published on the web: 15 February 2000.

Author: Richard Payling