angle of normal at P
Paraxial:
For 
rad (
), 
and 
to
%.
So for paraxial case, 
Snell's law: 
If slope of incoming paraxial ray 
(i.e.,
, then
Slope: 
Coordinate: y'=y (same point)
So we can write a matrix equation for the new slope and coordinate:
where 
is the refracting power
of the surface.
is the refraction matrix for surface 1 and transforms the incident
ray into the refracted ray.