## Friday 19 November 2021

### Speed of light in glass

I want to prove that light is a wave that propagates through a medium.  To highlight this point I want to consider the speed of light as it moves through glass.  The best explanation from the establishment about the speed of propagation through glass is in these two videos:

However, what they are describing is movement of the atoms in the glass.  Which to my mind means propagation through the medium.

## A better explanation

The speed of light is governed by two constants.  They are the permittivity of free space and the permeability of free space.  In addition to these two constants there are also relative permeability and relative  permittivity.

## Glass

I looked at the internet for the values of relative permittivity and permeability of glass, and found relative permeability to be between 5 and 10, so I used the value of 5 for the calculation. The given relative permittivity value was 3/8.  The following picture shows the results:

## Plastic

I have just done the same calculation for plastic.  I used a relative permittivity of 2.25 for polyethylene and a relative permeability of 1.  The result was 2 x 10^8 which according to the internet is correct!

## Water

Unfortunately, I could not do the same for water as I could not find any reliable information.

## Diamond

The speed of light in the Diamond is 1.24 x 10 ^ 8 metres per second.  To obtain this speed I used the relative permittivity of 5.8 and a relative permeability of 1.0037.  The Internet gave the range of 5.5 to 10 for the relative permittivity.