I understand that refraction is a bending of light that occurs when light passes between two mediums with different optical density. Because the light is represented as a wave-front, one side of the wave-front hits the medium first and slows down (or speeds up) first, which causes the wave to bend. This is the explanation I've always heard, and I never questioned it until recently...when I realized it's just glossing over the truth of the matter. The truth is the above explanation does not make sense unless the "rays" composing the wave have a cohesive force. This cohesive force would do the work of actually bending the wave front. Without a cohesive force, it would just change which photons are composing the wave front without any actual bending occurring. It is intuitive to think of a cohesive force existing which is why the commonly used example of a car that drives from pavement into sand will be turned. The cohesive force there is obviously the electromagnetic force creating molecular bonds between the atoms of the car. But photons have no mass and no charge, so what would give them cohesion?