# Snell’s Law Calculator

URL: https://speedycalculator.com/calculators/physics/snells-law/
Last updated: 2026-10-25

> The Snell’s law calculator finds the angle of refraction when light passes between two materials, n₁ sin θ₁ = n₂ sin θ₂, and the critical angle for total internal reflection.

## Inputs

- First medium
- Second medium
- Angle of incidence (°)

## Results

- Angle of refraction
- Critical angle

## Formula

```
n₁ sin θ₁ = n₂ sin θ₂
Critical angle = arcsin(n₂ ÷ n₁)
```

## Explanation

Light bends when it changes speed between materials, which is why straws look broken in water.

### How to use the Snell’s Law Calculator

- **First medium**
- **Second medium**
- **Angle of incidence**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Multiply n₁ by the sine of the incident angle, divide by n₂ and take the arcsine.

### Inputs explained

FieldWhat to enterFirst mediumChoose one: Air (1.0003), Water (1.333), Glass (1.52), Diamond (2.417)Second mediumChoose one: Air (1.0003), Water (1.333), Glass (1.52), Diamond (2.417)Angle of incidenceIn °

### Example values

How the main result changes when **Angle of incidence** changes and everything else stays at the example values above:

Angle of incidence (°)Angle of refraction (°)1511.19922.516.6893022.03737.527.1824532.0476040.532

## Worked example

Inputs — First medium: Air (1.0003); Second medium: Water (1.333); Angle of incidence: 30 °.

Results — **Angle of refraction = 22.037 °**.

## FAQ

**Why is there no refraction angle sometimes?**
Above the critical angle, light is totally internally reflected.

**How do optical fibres work?**
Total internal reflection keeps light inside the glass core.

**What formula does the Snell’s Law Calculator use?**
n₁ sin θ₁ = n₂ sin θ₂.