Interference Patterns in Caustics
![See accompanying text](https://webarchive.library.unt.edu/web/20201218022432im_/https://dlmf.nist.gov/9/SB2/g2.png)
A thin beam of light refracted by an irregularity in bathroom-window glass produces this image on a distant screen. The bright sharp-edged triangle is a caustic, that is a line of focused light. The oscillating intensity of the interference fringes across the caustic is described by the Airy function.