more details about Snell's law.
This equation applies to thin lenses. As lenses get thicker it becomes more complicated and this Snell's law does not exactly describe the refraction.
Snell's Law applies whenever a light ray cross the interface between two dielectric materials. It does apply to thick lenses.
Oh I guess I was confused about why it's different for thick lenses then as opposed to thin lenses. Is it that the thick lens applies it twice? Once for when the ray enters the lens and once for when it exits?
Actually, when a ray goes through a lens it refeacts twice. Once when entering the piece of glass, and once when exiting. This is the case whether the lens is thin or thick.