Ray diagram

A ray diagram (ray tracing) uses a set of drawing conventions and labels to visualise the path that a single ray of light takes in order to visualise what happens as it encounters different media, materials or objects.

Geometric optics treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.

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)