### Re: Why does light change speed when it enters a new medium?

Area: Physics
Posted By: Greg Dries, Senior Research Engineer,U. S. Steel Technical Center
Date: Mon Apr 28 10:42:42 1997
Area of science: Physics
ID: 861237548.Ph
Message:

To answer this question one needs to understand the nature of electromagnetic radiation (light) as well as the atomic structure of matter. Light is a traveling electromagnetic wave and therefore consists of an alternating magnetic field and an alternating electric field (each field inducing the other). James Clerk Maxwell discovered the laws governing electromagnetism which are the basis for our understanding of light. Consult any physics text book for details regarding Maxwell's work Maxwell's electromagnetic theory leads to the equation: c=square root of (1/(permeability constant times the permittivity constant), where c is the speed of light in a vacuum with the permittivity and permeability constants being for a vacuum as well. If the light wave is not traveling in a vacuum, these constants then become the permittivity and permeability values for whatever the media is, air, water, glass, etc. Understanding exactly what these constants are and what they relate to gets to the root of how electromagnetic radiation interacts with transparent materials. Without getting too deep, these constants are fundamental in much the same way as the gravitational constant relates to mass, except permittivity relates to electric fields and charged particles, and permeability relates to magnetic fields.

Because most all transparent materials are nonmagnetic, the permeability of transparent materials is essentially the same as the permeability of a vacuum, and therefore the speed of light in transparent media is dependent primarily on the permittivity of that media. The permittivity of solids is relatively large due to the interaction of the electromagnetic radiation with dipoles in the solid. These dipoles arise specifically from the types of atoms present and the nature of the chemical bonds among the atoms. The greater the density of the solid, the greater the number of dipoles per unit volume and the greater will be the change in the speed of light. Materials which exhibit the higher indexes of refraction produce a greater change to the speed of light within that particular solid. For the different types of glass, the atomic number of the cation constituents will control the index of refraction (lead-containing glass will have a much higher index than regular glass) because lead is a heavier atom. For plastics, the index of refraction is probably more a factor of the degree of cross-linking in the structure and the types of chemical bonds involved with the cross-linking, rather than the atoms themselves.

I hope this rather lengthy explanation helps. The science of materials is very fascinating and makes an excellent career choice. Look into it if your considering going onto college.

Sincerely
Greg Dries

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