Widefield Microscopy
Widefield microscopy is generally understood as the concept of fluorescence. This is defined by the spontaneous emission of light shortly after the excitation of a material. When light of a certain wavelength hits a molecule, photons are absorbed and electrons are lifted into a higher energy level. By falling back to the original level the resulting energy emits heat and a photon what is called the emission light.
More theoretical explanations are illustrated very clearly in the link below the University of Vienna.
Universität Wien - "Lichtmikroskopie online - Theorie und Anwendung"
Available in CFCI:
Zeiss Observer Z.1 (ApoTome I)
Zeiss Observer Z.1 (ApoTome II)
Zeiss Vert A.1
Zeiss Axio Zoom.V16
Zeiss Axioscan 7