PHOTONS

Film : Interference of Photons

Length(min.):13, Color: No, Sound: Yes, a PSSC Film John King, M.I.T.

In this film the wave and particle nature of light are exhibited in one experiment in which an interference pattern is examined with a photomultiplier.

It is recommended that this film be used only after viewing the film Photons. The film relates to the subject matter in Section 33-3 of the PSSC text.

Professor King describes the apparatus, which consists of an 8-foot-long box containing a weak light source. The light passes through a double slit, forming an interference pattern which is displayed visually. A photomultiplier connected to a sensitive ammeter is made to scan the pattern. The interference maxima and minima are clearly reflected in the meter readings. When the photomultiplier is connected to both an oscilloscope and a loud-speaker, the pulses seen on the oscilloscope screen correspond to the crackling of the loud-speaker. The pulse rate is seen to increase and decrease at the respective positions of the interference maxima and minima.

At a maximum of the interference pattern, the photomultiplier output current is measured to be 10exp -9 amp. Because the multiplier amplification is 10exp 6 this corresponds to an input current of 10 exp -15 amp or about 10 exp 4 electrons per second. Professor King points out that, on the average, only one electron is ejected for every 10 exp 3 photons incident on the photocathode. Thus, a current of 10 exp 4 electrons per second corresponds to about 10 exp 7 photons per second incident on the photocathode.

In 10 exp -7 seconds a photon travels about 100 feet. Therefore, it is argued that there is rarely more than one photon in the 8-foot-long apparatus. It is concluded that the interference pattern must be characteristic of individual photons, instead of an interaction between two or more photons.