Snell's Law & Refraction Calculator

Snell's Law Guide

Snell's Law

Snell's Law describes how light bends when passing between media with different refractive indices: n₁ sin θ₁ = n₂ sin θ₂. The refractive index n = c/v, where c is the speed of light in vacuum and v is the speed in the medium. Light bends toward the normal when entering a denser medium (higher n) and away from the normal when entering a less dense medium. At normal incidence (θ₁ = 0°), there is no bending regardless of the refractive indices.

Total Internal Reflection

When light travels from a denser medium (higher n) to a less dense one (lower n), there is a critical angle above which all light is reflected back into the denser medium — no refraction occurs. Critical angle θc = arcsin(n₂/n₁). For glass (n=1.5) to air: θc = arcsin(1.0/1.5) = 41.8°. For diamond (n=2.417) to air: θc = 24.4° — this is why diamonds sparkle so brilliantly, trapping and internally reflecting light from many angles.

Optical Fibres

Optical fibre communication relies entirely on total internal reflection. Light travels down a glass fibre and is totally internally reflected at every interface with the lower-index cladding surrounding the core. This allows light to travel around bends and over thousands of kilometres with minimal loss. The core has a slightly higher refractive index than the cladding. Modern fibre optic cables transmit data at near the speed of light with bandwidth far exceeding copper cables — they underpin

Everyday Refraction

Why a straw appears bent in water: light bends as it crosses the water-air interface, shifting the apparent position of the submerged part. Why swimming pools look shallower than they are: upward-moving light bends away from the normal at the water-air interface, making the bottom appear closer. Mirages: hot road surfaces create layers of different-temperature air with different refractive indices, causing total internal reflection of skylight to appear as a reflection on the road surface.