1.3 Photon Momentum

Einstein’s photon concept applies to all regions of the electromagnetic spectrum, including radio waves, X rays, and so on. A photon of any electromagnetic radiation with frequency f and wavelength λ has energy E given by Eq. (9.2). Furthermore, according to the special theory of relativity, every particle that has energy must also have momentum, even if it has no rest mass. Photons have zero rest mass. A particle with zero rest mass and energy E has momentum with magnitude p given by E = pc. Thus the wavelength λ of a photon and the magnitude of its momentum p are related simply by

p = (momentum of a photon) (9.5)

The direction of the photon’s momentum is simply the direction in which the electromagnetic wave is moving.

1.4 Phototube

Figure 9.7. Phototube

A phototube, also known as a photoelectric tube, is a light-sensitive electronic device. It is designed to emit an electric current when exposed to light or another form of electromagnetic radiation. The phototube has earned the moniker "electric eye," because of its use in a wide range of light-sensing applications. There are three categories of phototubes: gas, vacuum, and electron multipliers. The basic phototube is made up of a gas-filled or vacuum electron tube in which two electrodes, a cathode and anode, are mounted. A gas phototube utilizes argon, or some other type of inert gas, at a very low pressure.

The electron multiplier is a modification on the basic gas and vacuum phototube. In addition to two electrodes, this type of phototube is also equipped with a series of metal disks called dynodes. The dynodes are strategically arranged inside the electron tube to amplify the emission of electric current.

The surface of the cathode in a phototube is typically coated in cesium or some other type of photo-emissive material. This coating is extremely sensitive to light in the ultraviolet, near-infrared, and visible ranges of the electromagnetic spectrum. A phototube works by means of a process called the photoelectric effect, whereby light falling onto a photo-emissive surface causes electrons to be released from that surface. Through this process, when the surface of the phototube's cathode is exposed to light, it emits electrons. These electrons are attracted to the positively-charged anode, creating an electric current.

Phototubes can be divided into three general categories of use: control, measurement, and audio-visual reproduction. Under control come light-sensing applications such as burglar alarm systems, automatic door openers, and traffic light controls. Measurement functions include their use in color temperature pyrometers, while audio-visual reproduction applications include theater sound systems and pinball games.

In a burglar alarm system for example, a phototube and a beam of light are used as part of an electric circuit. The beam of light is directed onto the phototube, which is installed some distance away. When the beam of light is interrupted, as when a door or window is opened, the circuit is broken. This may happen, for example, if a burglar jimmies open a door. The broken circuit will, in turn, cause a relay to close, sounding the burglar alarm.

In recent years, photoelectric cells have been developed, which use employ semi-conductors rather than an electron tube. This has rendered phototubes obsolete in many cases. Electron multipliers, however, are still in use for a number of applications.