Question:How do we represent electromagnetic radiation in a diagram?
Usually we use rays and ray diagrams.
A ray diagram consists of a set of rays that show you where the electromagnetic radiation goes in that particular case.
A ray is just a straight line with an arrow head half way along showing the direction of the EM radiation.
Below are some simple examples of how rays can be used to show the passage of light through a couple of mirrors. A. is a periscope and B is a retroreflector.