PHOTONS

Film : Photons

Length(min.):19, Color: No , Sound: Yes , a PSSC Film John King, M.I.T.

In this film an experiment is performed to demonstrate the particle nature of light. The film relates to Section 33-1 of the PSSC text.

Professor King describes the apparatus he will use to demonstrate that light exhibits a particle-like behavior. A photomultiplier detects the very weak light used in the experiment. The operation of this device is outlined and the amplification is determined to be about 10 exp 6 (1 million)by measuring the output current and photoelectron current going into the first stage of the photomultiplier. The photomultiplier is connected to an oscilloscope, and pulses are seen on the oscilloscope trace. He shows that the pulses are due to the weak light shining on the photomultiplier, but that some pulses are due to background noise. To reduce this thermal background, the photomultiplier is cooled by a mixture of Dry Ice and alcohol.

The difference between the continuous wave model and the particle (photon) model for the transport of light energy is illustrated by an analogy to milk delivery. He shows that if the milk is to be delivered at the rate of one quart every ten seconds this can be achieved in either of two ways: (1) a pipe in which milk flows continuously at the uniform rate of one quart every ten seconds, or (2) a conveyor belt on which quart cartons of milk are randomly positioned so that on the average one quart of milk is delivered every ten secs.

In the first case then, there is a consistent 10-second delay before one quart of milk is delivered. However, in the second case, although on the average one quart (packaged) arrives every ten seconds, there is no consistent delay between the arrival of successive quarts; and thus some arrive at intervals of less than 10 seconds. It is this idea, of looking for the arrival of packages in less than the average time interval, that Professor King uses to find out whether light energy comes in packages (photons).

A beam of light shines on the photomultiplier through a hole in a disc. The light intensity is reduced with filters until the output current of the photomultiplier is only 3 x 10exp -16 amperes, implying that the photoelectron current is 3 x 10exp -16 amperes. This is equivalent to an average of one electron from the photocathode every 1/2000 of a second. The photomultiplier output is displayed on the oscilloscope and, with the disc spinning at a constant rate, it is determined that the light shines on the photomultiplier for 1/5000 of a second during each revolution. From the analogy using the flow of milk it is argued that a continuous transport of light energy would require 1/2000 of a second between pulses from a photoelectron; whereas a particle model would imply that at any instant during the 1/5000-second interval one might see a pulse from of a second photoeIectron, with the average rate still one pulse every 1/2000 of a second. The pulses are seen to arrive randomly during the 1/5000-second interval implying the particle nature of light.