# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Classical - Speed of Light in a Vacuum What is Light?  |  Wave's Family Members  |  Electromagnetic Waves How Do We Create Electromagnetic Waves?  |  Description of EM Waves Spectrum of Electromagnetic Waves  |  Visible Spectrum Speed of Light-First Try  |  Speed of Light in a Vacuum  |  Speed of Light in Matter
In the 19th century, people made discoveries in electricity and magnetism. After zillions of experiments, they discovered a consistent description of observed electromagnetic phenomena. The two most important physical laws discovered were:

1. the quantitative description of the force acting between two charges
2. the quantitative description of the force acting between two magnets (or equally, between two wires running current)

These two laws contain two new fundamental constants of nature, called permeability and permittivity of vacua. These two constants set the strength of the electric and magnetic forces. We will discuss the need for these constants in a while.

Later on, a further and final step was made when British physicist James Maxwell discovered a way to unify all the known electric and magnetic phenomena under one theory of electromagnetism. His equations were very rich in information and had incredible predictive power.

### Maxwell's Equations in a Vacuum

For example, you can easily (well, after doing some math, of course) read out from Maxwell's equations our wave equations for electromagnetic waves introduced on the page "What is light?". If you do this little exercise, you will discover that the parameter c in the wave equations is not a free parameter (as amplitude and frequency were) but can be calculated from the two fundamental constants of nature, the permittivity and permeability .

The numerical value of c is

c=299,792,458 m/s

where m/s is meters/second.
The physical meaning of this value is the speed of the propagating electromagnetic wave!

As soon as people realized that light is just an electromagnetic wave, it was possible to apply all of Maxwell's tools. Physicists then had a full description of the light in the classical (nonquantum) framework.

On the next page, I will discuss the speed of light when it propagates in matter.

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