Refraction
The bending of a wave when it crosses a boundary between media with different wave speeds. Snell's law: n₁ sin(θ₁) = n₂ sin(θ₂), where n is refractive index and θ is angle to the normal. Refraction occurs because wavelength changes (frequency is constant) when a wave enters a medium with different speed.
Formula
n₁ sinθ₁ = n₂ sinθ₂
Real World
Optical fibres carrying broadband internet (such as BT's full-fibre network) rely on total internal reflection inside a glass core — a consequence of refraction at the glass–cladding boundary.
Exam Focus
Always measure angles from the normal, not the surface; using the surface angle is the most common Snell's law mistake.
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