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Photoelectric cells were originally used to detect light, using a vacuum tube containing a cathode, to emit electrons, and an anode, to gather the resulting current. Today, these "phototubes" have advanced to semiconductor-based photodiodes that are used in applications such as solar cells and fiber optics telecommunications.

Photomultiplier tubes are a variation of the phototube, but they have several metal plates called DYNODES. Electrons are released after light strikes the cathodes. The electrons then FALL onto the first dynode, which releases more electrons that fall on the second dynode, then on to the third, fourth, and so forth. Each dynode amplifies the current; after about 10 dynodes, the current is strong enough for the photomultipliers to detect even single photons. Examples of this are used in spectroscopy (which breaks apart light into different wavelengths to learn more about the chemical compositions of star, for example), and COMPUTERIZED axial tomography (CAT) scans that EXAMINE the body.

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