Re: How fast does speed of light travel through solid, water, and air?

The speed of light in a vacuum is the universal constant c, which is
2.998 * 10^8 meters/second.

Now, in any medium (e.g. air, water, or glass, etc.), light will travel
slower than this. This is because light is essentially an electromagnetic
wave, and the transmission of this wave depends on the electrical
characteristics and density of the medium it is passing through. 

The 'index of refraction' (N) of a medium gives the average velocity (v)
of light in it by the relation:

     v = c/N

where c is the speed of light in vacuum, above. N is equal to 1 for a
vacuum, and will be greater than 1 for any other medium. Below are
the approximate indices of refraction of a few materials for visible light:

   Medium             N   
   ------          ------
    air            ~1.02
    ice             1.309
    water           1.333
    glass           1.50
    quartz          1.644
    diamond         2.417

You can look for other indices in books like the CRC Handbook of Physics,
or other sources. 

Keep in mind that these numbers are approximate, and that the index of
refraction of a material can change with temperature, pressure, and even
the wavelength of the light (which is how prisms work!).