How do fluorescence microscopes work?
Early in the 20th century, the phenomenon of fluorescence was applied to microscopy. Fluorescence is a luminescence phenomenon. Usually we see objects when light is reflected from them – the colour of an object depends on what wavelengths it reflects. With fluorescence, a photon (a light ‘particle’) of certain wavelength is absorbed by a molecule, and then re-emitted at a longer wavelength. Fluorescence is a very commonly used technique in biological imaging. Biological materials usually scatter a lot of light making it difficult to see beyond the surface of the cell. With fluorescence, the emitted light is always longer wavelength than the excitation light, so the light scattered from the cell surface can be separated from the emitted fluorescent light using dichroic mirrors that reflect the excitation light into the sample but let through the fluorescence light, making it possible to see structures inside the cell. Some biological materials are naturally fluorescent, but there are al
