Physics 2 · Unit 6: Geometric and Physical Optics · 14 min read · Updated 2026-05-11
Geometric Optics: Refraction and Reflection — AP Physics 2
AP Physics 2 · Unit 6: Geometric and Physical Optics · 14 min read
1. Law of Reflection and Index of Refraction★★☆☆☆⏱ 3 min
The most fundamental quantity in geometric optics is the index of refraction $n$, which describes how much slower light travels in a medium compared to vacuum. All angles are measured relative to the normal (perpendicular to the boundary), not the boundary itself.
n = \frac{c}{v}
When light crosses a boundary, its frequency $f$ does not change (set by the source), so wavelength changes proportionally to speed: $\lambda_n = \frac{\lambda_0}{n}$, where $\lambda_0$ is the wavelength in vacuum/air.
Exam tip: Always double-check whether a problem gives the angle relative to the boundary or the normal. If it's relative to the boundary, subtract from 90° before doing any calculations.
2. Snell's Law of Refraction★★★☆☆⏱ 4 min
When light transmits across a boundary from medium 1 to medium 2, it bends (refracts) because its speed changes. The relationship between incident and refracted angles is given by Snell's Law.
n_1 \sin\theta_1 = n_2 \sin\theta_2
A simple rule of thumb for bending direction: if $n_2 > n_1$ (light moves into a slower medium), light bends toward the normal. If $n_2 < n_1$, light bends away from the normal.
Exam tip: When asked for direction of bending, always compare the indices first: higher $n$ = slower speed = bend toward the normal. Don't guess from diagrams, which are often not drawn to scale.
3. Total Internal Reflection and Critical Angle★★★☆☆⏱ 3 min
Total internal reflection (TIR) is a phenomenon where all incident light reflects back into the original medium, with no refraction into the second medium. TIR only occurs when light travels from a higher index medium to a lower index medium ($n_1 > n_2$). If the incident angle is large enough, Snell's Law would require $\sin\theta_2 > 1$, which is impossible, so no refracted ray exists.
\sin\theta_c = \frac{n_2}{n_1} \quad (n_1 > n_2)
TIR is the operating principle behind fiber optic communications, diamond sparkle, and reflecting prisms in binoculars.
Exam tip: TIR can never occur when light moves from lower $n$ to higher $n$. Always check the direction of travel first before calculating critical angle.
4. AP-Style Additional Worked Examples★★★★☆⏱ 4 min
Common Pitfalls
Why: Problems often intentionally give angles relative to the surface to test convention knowledge
Why: Students memorize the formula but forget TIR only occurs when going from higher to lower index
Why: Students confuse wavelength and frequency changes, incorrectly assuming frequency scales with $n$
Why: Students mix up the relationship between $n$, speed, and bending direction
Why: Students mix up which index is which when memorizing
Why: Students confuse reflection geometry with refraction geometry