# Question #c3453

Sep 22, 2017

1 condition is the speed of light can is unique to the medium (i.e. light travels slower through water than air)
2nd I believe is Huygens' principle.
(Fermat's principle is a result, not a condition)

#### Explanation:

Refraction of light is dependent upon a speed change at an interface (a surface of two mediums). When going from a faster medium to a slower medium, the waves do not propagate as rapidly in the new medium and therefor do not diffract as much.

Christian Huygens' principle suggests that each point on a wave acts as a new wave source... think of a drop of water in a calm pond spreading outwards and that concept is happening everywhere on the wave. The waves in a slower medium are not able to spread out as rapidly and therefor the side closest to the fast medium is favored.

Picture holding long plank with Usain Bolt on the other end. Both of you are asked to sprint forward. As he runs faster than you, the board will tilt and your mutual running will bend instead of continuing straight forward. ...analogous to light waves at the border of the two mediums.