Light travels faster in the atmosphere than in water, because light refracts. How is this possible?

Answer

Light, like all EM (electromagnetic) waves, interact with matter in multiple ways, including

– gravity, or curvature of space due to the presence of mass. Let’s disregard this for a moment.

-exchange energy and momentum with electrically charged particles, such as the electron and the proton. These exchanges happen in “fixed packets of energy” called “quanta”. A quantum of light is called “photon”. For example, such exchanges can result in light being absorbed, created, or changed direction (Compton scattering).

Why does light travel slower in matter than in vacuum? There is a macroscopic property of materials called “permittivity” (http://en.wikipedia.org/wiki/Permittivity), and this is the parameter (in addition to the permeability: http://en.wikipedia.org/wiki/ Magnetic_permeability) that influences the speed of light: The following applies to almost all materials: the higher the permittivity, the lower the speed of light.

Permittivity is the ability of a material to polarize under the influence of an electric field, partially reducing the field within the material. Polarizing means that “loose” charge carriers (electrons, protons) in the material move freely with the electric field — and that electric field is the field of the EM (electromagnetic) wave itself.

So we’re there: the more “truly free” charges the material contains, the higher the permittivity and the lower the speed of light. However, by a large approximation.

Heavy atoms contain more electrons (and protons) per unit volume than light ones. The question is: are these electrons really freely polarizable? Moreover, the polarizability strongly depends on the frequency and wavelength of the EM waves.

A vulgarizing simplification of the above is:

-the light waves are slowed down because they interact (polarize…you can call that colliding…?) with the electrons and protons in matter.

-in general, the denser the matter, the more electrical charges in it, and indeed, the slower the light. But this rule is contestable. In addition, there is also such a thing as “permeability”.