Incident

Incident light refers to incoming light that is travelling towards an object or medium.

  • 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