Refraction, Snell's law and total internal reflection
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Question
What is refraction?
Answer
The **bending** of a wave as it crosses from one medium into another, caused by a **change in its speed** at the boundary.
Question
What does the refractive index n of a material tell you?
Answer
How much the material **slows light down**: **n = c ÷ v**. A bigger n means slower light and more bending (an optically 'denser' medium).
Question
State Snell's law.
Answer
**n₁ sinθ₁ = n₂ sinθ₂** — the indices and angles (measured from the normal) on each side of a boundary are linked this way. **Given** in the data booklet.
Question
Where are angles of incidence and refraction measured from?
Answer
From the **normal** — the line drawn at 90° to the surface — not from the surface itself.
Question
Light enters a denser (slower) medium. Which way does it bend?
Answer
**Toward** the normal — the angle gets smaller.
Question
Light enters a less-dense (faster) medium. Which way does it bend?
Answer
**Away** from the normal — the angle gets bigger.
Question
What is total internal reflection (TIR)?
Answer
When light hitting a boundary is **completely reflected back** into the medium it started in, instead of refracting through. None of it escapes.
Question
What is the critical angle θc?
Answer
The angle of incidence at which the refraction angle is exactly **90°**. Above θc you get total internal reflection.
Question
Formula for the critical angle?
Answer
$\sin\theta_c = \dfrac{n_2}{n_1}$ — derived from Snell's law by setting θ₂ = 90°. n₁ is the denser medium.
Question
What two conditions are needed for total internal reflection?
Answer
1) Light going from a **denser to a less-dense** medium, and 2) an angle of incidence **above the critical angle**.
Question
How do you find the speed of light in a medium from its index?
Answer
Rearrange **n = c ÷ v** to **v = c ÷ n**, using c = 3.0 × 10⁸ m s⁻¹.
Question
Most common refraction mistake?
Answer
Measuring the angle from the **surface** instead of from the **normal** — always use the dashed normal line.
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Full study notes for Refraction, Snell's law and total internal reflection
Topic 3.3 hub
Wave phenomena
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