Snell's law
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
N_1 sin(θ_1) = n_2 sin(θ_2); total internal reflection when sin(thetac) = n_2/n_1 and n_1 > n_2.
Snell's law in plain English
Snell's law explains why light bends when it crosses from one clear material into another — like how a straw looks broken where it dips into water. Each material has a number n (its refractive index) that says how much it slows light down, and the law n_1 sin(θ_1) = n_2 sin(θ_2) links the angle going in to the angle coming out, so light bends toward or away from the straight-through direction depending on which side is 'slower.' If light tries to leave a slower material for a faster one at too steep an angle, it can't escape at all and reflects entirely back inside — 'total internal reflection,' which kicks in past a critical angle where sin(θ_c) = n_2/n_1 (only possible when n_1 > n_2). That trapped-light trick is exactly how fiber-optic cables pipe internet signals around bends without leaking light out the sides.
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