# Snell's law

When light crosses between materials its direction bends: n1 sin theta1 = n2 sin theta2.

Physics (university) · Part 09: Induction, light and optics · Card 8 of 11 · Last updated 2026-10-19

## Analogy

A car turning when one wheel hits mud first.

## Example

**Air to glass**

1 x sin 30 = 1.5 x sin theta2
sin theta2 = 0.333
theta2 = 19.5 degrees

## In practice

Light entering glass (n = 1.5) from air at 30 degrees bends to about 19.5 degrees.

## Practice

### Flashcards

- **State Snell's law.** n1 sin theta1 = n2 sin theta2.

### Interview questions

**Light enters a glass block at 40 degrees. The glass has refractive index 1.5. Find the angle in the glass and explain total internal reflection.**

- Use Snell's law
- sin theta2 = sin 40 / 1.5
- Critical angle for glass to air is 41.8 degrees

## Questions people ask

### What is the refractive index?

The refractive index n is the ratio of the speed of light in a vacuum to the speed in the material, n = c/v. Water has n of 1.33 and glass about 1.5. A higher n means slower light and more bending toward the normal.

### Why does a straw look bent in water?

Light from the part under water bends as it leaves into air, so rays seem to come from a different place. Your brain traces them straight back, making the submerged part look raised and the straw look broken at the surface.

## Related concepts

- [Total internal reflection](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-9)
- [Electromagnetic wave](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-6)
- [Electromagnetic spectrum](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-7)
- [Thin lens equation](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-10)

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