Fluorescence

A chemical is said to be fluorescent if it absorbs light of one wavelength (excitation light) and emits light of another, longer specific wavelength (emission light). Shorter wavelengths of light contain more energy than longer wavelengths of light. Fluorescent molecules are useful in microscopy because they can be attached to other molecules that have specific affinity for cell structures or components (e.g. fluorescent labeled DNA probes - see Direct or Indirect Labeled DNA Probes). When viewed with a properly equipped fluorescent microscope, one sees fluorescent signals wherever the target structure or component is present in the cell. In modern fluorescent microscopes, only the light emitted by the sample is used to form an image - the exciting wavelengths of light are blocked by special filters between the microscope lens and the viewer's eye or camera.

Excitation

Emission