Snell's Law (Laws of Refraction) (n1 sin θ1 = n2 sin θ2)

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n1 sin θ1 = n2 sin θ2

What each symbol means

SymbolWhat it stands for
n_1Refractive index of first medium (incident side)
n_2Refractive index of second medium (refracted side)
\theta_1Angle of incidence with the normal (rad or deg, consistent)
\theta_2Angle of refraction with the normal

When to use this

Isotropic media; monochromatic light at a sharp plane interface; valid for specular refraction. Angles measured from the normal.