Light behaving like a particle
Teaching Guidance for 14-16
No sooner do students see that light has a wave property (and have measured its wavelength) then this story is upset with further demonstrations that light has a particle property; it packages its energy in small quanta. The idea that radiation packages its energy in quanta proportional to frequency first arose in Planck’s mind when trying to fit the theoretical prediction for the energy distribution in the spectrum of a perfect radiator with the experimental results. The variation of the specific heat of materials with temperature also appeared to require a quantum rule. The photoelectric effect appeared to be pointing in the same direction when Einstein applied his clear vision to it in 1905 and was awarded the Nobel Prize for his efforts.
It is assumed that pupils have seen photocells at work in electric or electronic circuits where light releases a horde of electrons from a sensitive surface in a vacuum and the horde acts as a current to do jobs for us. That might be called the ‘wholesale photoelectric effect’. In this, light ‘flicks’ electrons out of a metal, ultra-violet light tearing them out with the crack of a whip and X-rays hurling them out. This strange interchange between radiation and electrons throws much ‘light’ on the micro-physical world.
A Geiger-Müller tube responding to gamma rays is demonstrating the photoelectric effect of those very energetic photons. However, the random counting is due to the random instability of the parent radioactive nuclei, not the effect of photons arriving at random from a steady stream of radiation. But if you shine a steady stream of ultra-violet light or light from a match onto a Geiger-Müller tube, with a thin mica window, then the Geiger-Müller tube will show random counts. A sheet of glass placed between the light source and the Geiger-Müller tube will show that it is not the visible light which is the active agent.
This experiment suggests some of the photo-electric effect story, but it does not show that the negative electricity is coming out in particles: electrons. It also does not show that light is arriving in bundles of energy: quanta. It only suggests that there is some connection between the wavelength of the light and its efficacy in ejecting negative charge.
More complex experiments, or perhaps a film, are needed to show:
- photons arriving one by one
- that the particles ejected are electrons with the usual value of e / m
- that the electrons emerge with a given illumination, with a variety of speeds, the slower ones having probably lost energy by travelling through the outer layers of the metal
- that with light of a given frequency, all the electrons ejected have the same maximum energy. This is the basis of Einstein’s equation, Eelectrons = hflight − Φ, where Φ is the ‘work function’ and h is the Planck constant.
- that the maximum energy of the ejected electrons is determined by the frequency of the light used and not by its intensity. Brighter light only produces more electrons and not faster ones
- that when the light is first turned on there is no delay in the production of electrons as one would expect if a continuous stream of light had to build up enough energy in the metal to eject each electron in turn. This is especially an impressive story with weak light. Sometimes an electron is ejected early, sometimes it may be later and so we are forced to conclude that the arrival of quanta is random in time.