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On the syllabus: GCSE PhysicsA-Level Physics
A straw in a glass of water looks snapped in two — but touch it and it's perfectly whole. That same bending trick lets hair-thin glass fibres carry every YouTube video, every WhatsApp message, and every Zoom call across the ocean floor at nearly the speed of light without losing a single bit. How can light bending in a glass of water be the same phenomenon that carries the entire internet?
A straw in a glass of water looks snapped in two — but touch it and it's perfectly whole. That same bending trick lets hair-thin glass fibres carry every YouTube video, every WhatsApp message, and every Zoom call across the ocean floor at nearly the speed of light without losing a single bit. How can light bending in a glass of water be the same phenomenon that carries the entire internet?
When light crosses from one transparent material into another it changes speed — slowing down in glass or water, speeding up back in air. Any wave that changes speed at a boundary also changes direction. That bending is called refraction. It's why a pool looks shallower than it is, why mirages shimmer above hot roads, and why optical fibres can guide light around corners with no loss at all.
Every transparent material has a refractive index n = c/v, where c is the speed of light in vacuum and v is the speed in the material. Air: n ≈ 1.00. Water: n = 1.33. Glass: n ≈ 1.5. Diamond: n = 2.42. The higher n is, the slower light travels and the more it bends. When light tries to leave a denser medium (high n) at a steep angle, it cannot escape — this is Total Internal Reflection (TIR). Optical fibres exploit TIR in a glass core surrounded by slightly less-dense cladding to pipe light signals over thousands of kilometres.
Snell's Law connects angles and refractive indices. For TIR, set θ₂ = 90° to find the critical angle: sinθ_c = n₂/n₁. Glass-to-air: sinθ_c = 1.0/1.5 → θ_c ≈ 41.8°. Any angle inside the glass steeper than 41.8° gives perfect reflection back into the glass — zero loss. Diamond cutters exploit TIR by designing facets so that almost all entering light bounces repeatedly inside before exiting through the top face, maximising sparkle.