# Fresnel biprism

> A shallow double wedge splits one beam into two that overlap on the screen like twin sources.

![Fresnel biprism](https://www.loomrume.com/optics/stills/fresnel-biprism.png)

**Augustin-Jean Fresnel** · 1816, Paris — prize memoir on diffraction

A shallow prism, really two weak prisms back to back, splits one beam into two that overlap as if they came from a pair of virtual slits. Fresnel used that to get interference without cutting two holes in a card — useful when critics said Young’s [slit](https://www.loomrume.com/examples/single-slit.md) demos were just “edges doing something odd”.

The same year he won the Academy prize. Poisson, arguing against him, noticed the theory predicted a bright spot in the centre of a circular shadow. Arago looked, and the spot was there. This bench is the biprism half: two bundles on the screen, no hole in a card.

## Sources

- [Fresnel biprism](https://en.wikipedia.org/wiki/Fresnel_biprism)
- [Augustin-Jean Fresnel](https://en.wikipedia.org/wiki/Augustin-Jean_Fresnel)
- [Arago spot](https://en.wikipedia.org/wiki/Arago_spot)

## Links

- [HTML](https://www.loomrume.com/examples/fresnel-biprism)
- [Open in Studio](https://www.loomrume.com/experiments/new?template=fresnel-biprism)
- [llms.txt](https://www.loomrume.com/llms.txt)