The Photon Revealed

Ryan Clouter, 15 years ago

Grade:12

1 Answers

Sachin Tyagi

31 Points

15 years ago

Curiously enough, although the photon was proposed by Einstein in 1905, a strong case can be made that truly convincing experimental evidence for it emerged only in 1974 and especially in 1986, with the work that we describe in this section. What we seek is an experiment that not only supports the photon concept but that cannot also be explained by the wave concept.

Figure suggests the broad outlines of a possible experiment. Light from source S falls on beam splitter B.A beam splitter is an optical device that splits a beam of light into two sub beams, a transmitted beam (beam X) and a reflected beam (beam Y), each of these sub beams having of photons, a photon approaching a beam splitter has 50% chance of being transmitted and a 50% chance of being reflected.

D₁ and D₂ in the figure are photoelectric light detectors that respond, respectively, to beams X and Y. the light source S is deliberately made feeble enough so that, on average, the total light energy in the apparatus at any given time is no greater than that of a single photon. Under these conditions each detector output will be a serried of discrete electronic pulses.

If the photon model for light is correct, we expect that if a pulse appears at the output terminal of either detector at a given instant, no pulse will appear at the output terminal of the other detector at that same instant. Such anticoincidence, as they are called, can be detected and recorded electronically by arrangements not shown in the figure.

Early experiments of this kind showed no convincing pattern of anticoincidence and thus no confirmation of the photon model for light. Later it was shown that these negative results could be accounted for by small fluctuations in the intensity of the light source, among other factors. Thus the experiment, as we have described it so far, is not yet a convincing test for the presence or absence of photons.

Note that the apparatus of the figure can not reveal the wave nature of light. Although the incident light beam is split into two sub beams—just as in the double-slit experiment—these beams enter separate detectors and are never recombined. Thus they cannot interfere with each other and cannot possibly produce the interference effects that are the hallmark of the wave model for light.