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Refraction of light

When light reaches an interface between two media, for example, between air and water, or air and glass, some of the light is reflected and some propagates into the second medium. If the second medium is transparent, like glass or water, then most of the light will propagate through the new medium with a change in direction and speed and only a little will be reflected. The change in direction of a light bean at the boundary of the two media is called refraction.

The details of the interactions between incident light and matter (for example, how much light is reflected, refracted or absorbed) depend on the interactions between the electromagnetic nature of light and the electric charges (free or bound, coupled or un-coupled, positive or negative) that comprise the target matter.

It is a lengthy subject and one invariably dealt with by approximations and idealisations. These approximations and idialisations may not always be satisfactory for a pure scientist, but they are actually very helpfull and usually necessary in order to get some basic and usefull understanding. Useful in the sense of enabling us to predict what is going to happen in a specific case. There are many way's for approaching this subject and many books even many good books have been written on the subject. Should you go searching in books on optics and optical physics you will find they fall, more or less, into four distinct approaches:

Approximation used for Light

Approach

Approximation used for Matter


Classical
waves
particles

Semi-classical
waves
quantised

Semi-quantum
quantised
particles

Fully quantum
quantised
quantised

Two extremes of matter are ideal metals (dominated by free electrons) and perfect insulators or dielectrics (dominated by bound electrons). These are often approximated by their average properties such as conductivity (or resistivity), permittivity, permeability, etc.

Previously, we discussed reflection from an ideal metal. The metal surface forms a potential barrier that promotes reflection. But it is a feature of quantum mechanics that particles can penetrate barriers, the intensity of radiation decaying exponentially into the metal. Hence, if we make a metal film sufficiently thin, some light will penetrate the film and be detected on the other side.

Insulators, like glass, on the other hand, are dominated by bound charges. The wavefunction of the incoming photon interacts with the electron wavefunctions in the insulator and causes an oscillation of the centre of gravity of the charge density in the substance.