Understanding basic microscopy starts with understanding basic optics.
To do that we will need to know a little bit about light.
Light, as we know, is an electromagnetic wave. An electric field varying in space and time creates a magnetic field that varies in space and time. A magnetic field that varies in space and time creates an electric field that varies in space and time. These varying electric and magnetic fields can sustain each other. Such waves do not require a medium to travel. This is a very simple way of looking at electromagnetic waves such as light. Waves interact with matter in a complex way. For electromagnetic waves, the laws governing their interaction is described by Maxwell's Equations.