Snell's Law describes how light bends when passing from one medium to another. It states that n1·sin(θ1) = n2·sin(θ2), where n1 and n2 are the refractive indices of the two media and θ1, θ2 are the angles measured from the normal to the interface.

What is total internal reflection?

When light travels from a denser medium to a less dense medium (higher n to lower n) and the incident angle exceeds the critical angle, all light is reflected back into the first medium — none passes through. This is total internal reflection, and it is the principle behind fiber optic cables.

What is the refractive index?

The refractive index (n) is the ratio of the speed of light in a vacuum to the speed of light in a given medium. Air ≈ 1.000, water ≈ 1.333, standard glass ≈ 1.520, and diamond ≈ 2.417.

Why are angles measured from the normal?

Optics conventions measure angles relative to the normal (a line perpendicular to the surface at the point of incidence), not the surface itself. This makes Snell's Law consistent for any orientation of the interface.

What does an incident angle of 0° mean?

An incident angle of 0° means light hits the surface straight on (perpendicular). In this case sin(0°) = 0, so the refracted angle is also 0° — light passes straight through without bending regardless of the media.

Snell's Law Refraction Calculator

Name: Snell's Law Refraction Calculator
Availability: InStock
Author: Nham Vu

Enter the incident angle and refractive indices of two media to calculate the refracted angle using Snell's Law.

Inputs

Incident Angle (θ₁)

degrees

Medium 1 (incident)

Medium 2 (refracted)

Refracted Angle (θ₂)

—

Critical Angle

—

for total internal reflection

n1 · sin θ₁

—

must equal n2 · sin θ₂

Ray Diagram

Formula: n₁ · sin(θ₁) = n₂ · sin(θ₂) → θ₂ = arcsin(n₁/n₂ · sin(θ₁))

Summary