Typing the phrase "definition of fluorescence" into any search engine and you will get the following phrase, or something very similar;
"The emission of visible or invisible radiation from certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light."
How does this definition relate to fluorescence microscopy (Figure 1)? The "incident radiation of a shorter wavelength" is simply the light source used to "excite" the fluorophores in a sample. This light can include the visible spectrum, ultraviolet (UV) and infra-red (