Light speeds

The speed of light is different in different media doesnt concnern with relativity. In fact, classical electromagnetic theory could be used to explain such phenomenon.

James Clark Maxwell, in the late 19 Century, combined the theories in electromagnetism into a set of 4 differential equations (now known as the Maxwells equations). From his equations, the speed of electromagnetic waves in vacuum was evaluated and found to be equal to 3 x 10^8 m/s, the same as the speed of light obtained from measurements. From this result, the nature of light was known. Light is electromagnetic wave.

In Maxwells equations, the speed of light depends on the permeability (the magnetic property) and the permitivity (the electric property) of a material in which light passes through. Generally, substances normally have permitivity and permeability higher than vacuum has, hence the speed of light is slower in most substances.

Simply speaking, when light enters a susbtance, the electric and magnetic field components of the light wave cause molecules (in the form of dipoles, i.e. molecules with positive charge at one end and negative charge at the other end) to oscillate. The process involves absorption and re-emission of electromagnetic radiation. The light wave thus travels [from dipoles to dipoles], making it propagates through the substance with a speed much slower than that in vacuum.

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