Optical Absorption in Metals

OPTICAL ABSORPTION IN METALS

We know that when a metal is exposed to light (example sunlight) then the metal gets heated after some time. The reason behind this is as follows.

In metals there are large number of free electrons, these electrons interact with the optical electric field. Due to this field the motion of electrons are damped by collisions with the vibrating lattice of the material and hence a part of energy is dissipated in the form of heat.

Skin depth and skin layer: When light passes through a metal, its intensity decreases with respect to the distance through which it propagates through the metal. Therefore the energy of the light decreases due to the absorption of light by free electrons.

Thus the skin depth is the maximum distance upto which an electro-magnetic wave (light) can travel thereby producing conduction current due to the interaction of light with free electrons. The conduction current which passes upto a layer is called Skin layer. (Fig. 4.8).

Note: The conduction current due to light will produce heat only upto skin layers. Below the skin layer the heating is produced due to thermal conduction only.