Certain specimen materials emit energy detectable as visible light when irradiated with the light of a specific wavelength. The sample might be fluorescing in its natural form, or it can be treated with fluorescing chemicals.
The fluorescence microscope has a filter that only lets through radiation with the desired wavelength that matches the material that is fluorescing.
Radiation collides with the atoms in the specimen and electrons are excited to a higher energy level.
As these atoms relax to a lower level, they emit light. In order to become visible.
This emitted light is separated from the much brighter excitation light in a second filter.
The fluorescing areas can be observed in the microscope and shine out on a dark background with high contrast.
The image above is stained plant tissue that was captured using a fluorescence microscope by Fernan Federici for Cambridge University. Via biovisual