- Incident light refers to light travelling towards something but is often visualised as a single ray.
- A ray diagram (or wave diagram) uses drawing conventions and labels to visualise the path that a single ray (or wave) of light might take as it encounters different media, materials or objects.
- An incident ray (or wave) and its direction of travel is usually shown on a diagram as it propagates through a first medium, prior to encountering a second medium.
https://en.wikipedia.org/wiki/Ray_(optics)#Interaction_with_surfaces