Plain-English Meaning
When light moves from air into water (or glass), it bends. The bending follows a simple rule — Snell's law — that involves refractive index n (how much the medium slows light) and angle from the perpendicular (normal). Total internal reflection happens when light tries to escape at too steep an angle.
If a waves question feels messy, sketch the geometry or phase relationship first, then return to the algebra.
Deeper Explanation
n = c/v (refractive index). Snell's law: n₁ sinθ₁ = n₂ sinθ₂ (angle measured from normal). Critical angle: θ_c = arcsin(n₂/n₁) — beyond this, total internal reflection occurs. Applies to all waves; for sound, n = c₁/c₂.
Worked Example
Problem: Light travels from water (n=1.33) to glass (n=1.5) at 45°. Find refracted angle.
- n₁ sinθ₁ = n₂ sinθ₂
- 1.33 × sin45° = 1.5 × sinθ₂
- sinθ₂ = 1.33 × 0.7071 / 1.5 = 0.9404/1.5 = 0.627
- θ₂ = arcsin(0.627) ≈ 38.8°
Result: θ₂ ≈ 38.8° (light bends toward normal as it enters denser glass)
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At A Glance
Category: Waves & Optics
Levels covered: High School, College, Masters, PhD
Best use: Start with the formula meaning, then move to the worked example and quiz so the equation turns into a tool instead of a memorised line.