Is it farther or further?
An electrical device designed for the detection of weak electromagnetic radiation, usually light, by amplifying the energy of the photons that strike it into stronger electrical signals. Photomultipliers are used in night-vision technology and in telescopes to detect light not strong enough to be visible by the unaided eye. ◇ The most common photomultiplier is the tube photomultiplier; it exploits secondary emission of electrons in a vacuum tube in the manner of an electron multiplier. When radiation strikes the cathode of a tube photomultiplier, electrons called photoelectrons are emitted and attracted to positively charged electrodes called dynodes. When they collide with the dynode, more electrons are released; these are in turn attracted to another dynode at a higher voltage to release yet more electrons, and so on. At the end of this process, there is a current flow at the anode that is strong enough to be easily detected.