Photons (Light) and Their Psychological Impact |
Although interferenceis intrinsically a classical wave phenomenon, the superposition principle whichunderlies all interference is also at the heart of quantum mechanics. Feynmanhas referred to interference as really “the only mystery” of quantum mechanics.Furthermore, in some interference experiments we encounter the idea of quantumentanglement, which has also been described as really the only quantum mystery.Clearly interference confronts us with some quite basic questions ofinterpretation. Despite its long history, going back to Thomas Young at thebeginning of the 19th century, optical interference still challenges ourunderstanding, and the last word on the subject probably has not yet beenwritten. With the development of experimental techniques for fast and sensitivemeasurements of light, it has become possible to carry out many of the Gedankenexperiments whose interpretation was widely debated in the 1920s and 1930s inthe course of the development of quantum mechanics. Although this articlefocuses entirely on experiments with light, interference has also been observedwith many kinds of material particles like electrons, neutrons, and atoms. For a given metal, there exists a certain minimum frequency of incidentradiation below which no photoelectrons are emitted. This frequency is calledthe threshold frequency. Increasing the frequency of the incident beam andkeeping the number of incident photons fixed increases the maximum kineticenergy of the photoelectrons emitted. The number of electrons emitted alsochanges because the probabilitythat each impacting photon results in an emitted electron is a function of thephoton energy. However, if just the intensity of the incident radiation isincreased, there is no effect on the kinetic energies of the photoelectrons.Ifthe photon energy is too low, the electron is unable to escape the material.Increasing the intensity of the light increases the number of photons in thebeam of light and thus increases the number of electrons excited but does notincrease the energy that each electron possesses. The energy of the emittedelectrons does not depend on the intensity of the incoming light (the number ofphotons), only on the energy or frequency of the individual photons. It isstrictly an interaction between the incident photon and the outermost electron.