Ray diagram

A ray diagram (ray tracing) uses a set of drawing conventions and labels to visualise the path that rays of light take in order to understand what happens as they encounter different media, materials or objects.

Ray diagrams are used in geometric optics which treats light as rays that travel in straight lines and change speed and/or direction when they encounter different transparent media.

The aim of a ray diagram is to demonstrate optical phenomena such as absorption, dispersion, polarization, reflection, refractionscattering and transmission.

https://en.wikipedia.org/wiki/Ray_tracing_(physics)