Refraction
   When light travels from one medium to the other, part
of the light is refracted into the second medium.  The
speed of light is changed, which causes the direction of
light to be changed.   The refracted angle will be
      sin q2  =  sin q1  ( c1 / c2 )
 

Snell's Law
   The ratio of the sine of the incident angle to the sine of the refracted angle is a constant.
   sin q1 / sin q2  = constant .
Because c1 / c2 = ( c / n1 ) / ( c / n2 ) = n2 / n1     then  Þ  sin q1  / sin q2 = n2 / n1
The constant is  n2 / n1