# Double-slit interference

Light through two narrow slits makes bright and dark fringes. Fringe spacing is y = lambda L / d.

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

## Analogy

Two sets of ripples overlapping on a pond.

## Example

**Fringe spacing**

y = lambda L / d
= 600e-9 x 2 / 2e-4
= 6e-3 m

## In practice

600 nm light, slits 0.2 mm apart, screen 2 m away: fringes 6 mm apart.

## Practice

### Flashcards

- **What produces bright fringes in the double slit?** Waves arriving in step, with path difference a whole number of wavelengths.

## Questions people ask

### Why did the double-slit experiment matter?

In 1801 Thomas Young used it to show that light makes interference fringes, which only waves can do. This supported the wave theory of light. Later the same experiment with single electrons or photons, one at a time, showed wave behaviour in quantum physics too.

### What happens to the fringes if I use red light instead of blue?

Red light has a longer wavelength than blue, and fringe spacing is lambda L over d. So with red light the fringes are further apart. White light gives a bright centre with coloured edges, because each colour has fringes at different spacings.

## Related concepts

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

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