# Total internal reflection

Light going from a dense to a less dense material reflects completely beyond the critical angle, sin theta c = n2/n1.

Physics (university) · Part 09: Induction, light and optics · Card 9 of 11 · Last updated 2026-10-19

## Analogy

The reason diamonds sparkle and optical fibres carry signals.

## Example

**Critical angle**

sin theta c = 1 / 1.5
= 0.667
theta c = 41.8 degrees

## In practice

For glass to air the critical angle is about 41.8 degrees.

## Practice

### Flashcards

- **When does total internal reflection happen?** When light goes from higher to lower refractive index beyond the critical angle.

### 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

### How does an optical fibre work?

Light enters a thin glass core and strikes the core wall beyond the critical angle, so it reflects completely each time. It zig-zags along the fibre with almost no loss for kilometres. Fibres carry internet data as pulses of light.

### Why can't total internal reflection happen from air to glass?

It needs light to move from a material with higher refractive index to a lower one. Going from air (n = 1) to glass (n = 1.5), the bent ray always exists, because sin theta2 would be less than 1. There is no critical angle in that direction.

## Related concepts

- [Snell's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-9/physics-university-part-9-concept-8)
- [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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