Huygens’ principle has been used to make predictions about the
position of a wavefront after waves have crossed a boundary
between
two different media. 



The diagram below results from the application of the principle
to waves moving from a region where the velocity is v_{1} to
a different medium where the velocity is v_{2} (in this
case, v_{2}<v_{1}) 





Angles ABC and ADC are 90°. 



Therefore we can write 



which gives 





which means that, for a given pair of media, the ratio, sinθ_{1}
to sinθ_{2}
is a constant, for a given pair of media (equal to the
ratio of the velocities of light in the two media). 

This relation is called Snell’s law after the Dutch astronomer
Willebrord Snellius (or the SnellDescartes
law) of refraction. 

It is especially easily verified by experiment using
light passing from, for example, air to a glass prism. 



The constant is called the refractive index (symbol, n_{12})
for waves passing from medium 1 to medium 2 

and, as the path of the waves is reversible, we can write 





The angle between a normal line and the direction of
propagation in medium one is called the angle of incidence
(θ_{1} in the diagram
above). 

The angle between a normal line and the direction of propagation
in medium two is called the angle of refraction (θ_{2}
in the diagram above). 



Snell's law: 

The sine of the angle of incidence divided by the sine
of the angle of refraction is equal to a constant called the
refractive index, n, of the two media. 


When considering light,, if medium 1 is a vacuum (or air) then
the ratio v_{1}/v_{2}
is called the absolute refractive index of medium 2. 
