What causes light to refract
Anything it passes through. Everything has a index of refraction
All of the answers I see here are the results of refraction rather than the cause. What I want to know is what are the physical properties of the various mediums such as glass, plastic, diamond that makes the light bend at different angles. Index of refraction is not a cause, but is only a measure that we have invented.
Refraction happens when light is going from one medium to another. If light moves through one medium at a faster rate than another (and it is not passing through the medium at a plane perpendicular to the plane between the two mediums - i.e. head on) then it will refract. This is caused by part of the light wave (for example, a top half) hitting the new medium before the other part. This means that part of the wave starts moving faster or slower than the other, causing it to turn/change direction. Light can also refract based on time. For example, when going by a large gravitational field. This is very similar. Instead of the light entering a new medium, it is passing through space at two different rates of time, causing it to turn.
When a light passes from one optical medium to another of different optical densities, then it will refract. The reason is that, the frequency 'v' of light does not change, but the wavelength 'λ' will change. Since, c = vλ, c will change accordingly. This change in speed of light causes it to refract.
