5.2 Refraction, Snell's Law, and Total Internal Reflection

Refraction bends light

Refraction is the bending of light as it crosses a boundary between media with different indices of refraction (n). Light slows down entering a denser medium (higher n) and bends toward the normal; it speeds up and bends away from the normal entering a less dense medium. This is described by Snell's Law: n₁sinθ₁ = n₂sinθ₂, where angles are measured from the normal.

Index of refraction

The index of refraction n = c/v compares the speed of light in a vacuum (c) to its speed in a medium (v). Since v ≤ c, n ≥ 1 always. A larger n means light travels slower in that medium and bends more sharply toward the normal when entering it from a less dense medium.

Total internal reflection

When light travels from a higher-n to a lower-n medium, at large enough angles it can't refract out—it fully reflects instead. This is total internal reflection (TIR), occurring beyond the critical angle θc, where sinθc = n₂/n₁ (n₁ > n₂). TIR only happens going from denser to less dense media and underlies fiber optics.

Key terms

Refraction

The bending of light as it passes between media with different indices of refraction, due to a change in speed.

Snell's Law

n₁sinθ₁ = n₂sinθ₂; relates angles of incidence and refraction to the indices of refraction of two media.

Index of refraction (n)

n = c/v, the ratio of light's speed in vacuum to its speed in a medium; always ≥ 1.

Critical angle (θc)

The angle of incidence at which the refracted ray travels along the boundary (90°); sinθc = n₂/n₁, requires n₁ > n₂.

Total internal reflection

Occurs when light hits a boundary from a denser to less dense medium at an angle greater than the critical angle, causing 100% reflection.

Try an AP-style question

Light passes from air into glass. Which way does it bend?

1. Away from the normal
2. Parallel to the surface
3. Toward the normal
4. It doesn't bend