Snell's Law
Definition
The bending rule for light entering a new material — it bends toward the normal when slowing down.
Technical definition
n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. It follows from Fermat's principle of least time.
Example
Light entering water (n = 1.33) from air at 45° bends to about 32° — which is why a straw looks broken in a glass.
Common confusion
Snell's law uses angles from the NORMAL, not the surface. Measuring from the surface is the number-one student error in refraction problems.