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Refraction: Snell's Law
Huygens’ principle has been used to make predictions about the position of a wave-front 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 v1 to a different medium where the velocity is v2 (in this case, v2<v1)  
   
 
Angles ABC and ADC are 90°.  
   
Therefore we can write  
   
 
which gives  
 
   
which means that, for a given pair of media, the ratio, sinθ1 to sinθ2 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 Snell-Descartes 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, n12) 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 (θ1 in the diagram above).  
The angle between a normal line and the direction of propagation in medium two is called the angle of refraction (θ2 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 v1/v2 is called the absolute refractive index of medium 2.   
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