ABSTRACT
In 1904 the German microscopist August Köhler was studying the use of ultraviolet light for increasing the resolution of the microscope. He noticed that some objects emitted visible light when irradiated with invisible short-wave ultra-violet. This emission of light is the phenomenon now called fluorescence. The absorbed radiation interacts with some molecules; a photon, absorbed by an atom of the material, causes an electron to move from the lowest energy level (the ground state) to a higher level (an excited state). This is an unstable situation and after a very short time (in the order of 1 × 10−9 sec) the electron spontaneously returns to the ground state, with the emission of a photon (see Figure 2.8 ). A related phenomenon which has not been adequately exploited in microscopy occurs when the time for the electron to return to the ground state is measured in seconds; this is called phosphorescence.
