1. kittykat Millikan and the photoelectric effect.

2. The photoelectric effect is the phenomenon in which electromagnetic radiation in the optical region of the spectrum – ie good old light – is shone on a metal surface and it causes electrons to be emitted from the said metal surface. A similar idea is the effect when a metal is heated to a high temperature and emits electrons. This effect was used a lot in the thinking behind “thermionic valves” which were used a lot in early to late 1900s. To try to understand the photoelectric effect, the light shining on the metal surface is thought of as an enormous number of photons. If the light is of one wavelength (monochromatic) then the photons will all have the same frequency, and thus – according to quantum theory – the same energy. This is in contrast to previous ideas of electromagnetic radiation, which viewed it as continuous wave.

3. The energy of the wave would depend on the amplitude of the oscillation of the continuous wave. The more intense the beam, the larger the oscillation amplitude. Light shining on a metal surface with a particular frequency has a particular energy depending on the frequency through the equation. The letter h is used for Planck’s constant. The idea was that the more intense the beam could be made, the more electrons would be emitted. Millikan’s experiment disproved that notion. Will that photon energy be enough to “pop” an electron out of the metal ? When the electron does its bit, it seems to vanish (annihilated ?). Its bit is to try to liberate an electron at least, and maybe give it some kinetic energy as well. Millikan and the photoelectric effect.

4. When the photon does its bit, it seems to vanish (annihilated ?). Its bit is to try to liberate an electron at least, and maybe give it some kinetic energy as well. If the photon has just enough energy, as determined by its FREQUENCY, to liberate an electron, then that is called the THRESHOLD frequency. Frequencies lower will have too little energy to do anything, and so there will be no photoelectric effect. (Presumably, the sad photon is saved from being annihilated, too.) Frequencies threshold will have enough energy to liberate an electron. Frequencies higher will have more energy and be able to liberate more electrons AND give them kinetic energy. Electrons are contained in all substances, and how they are contained is very complicated indeed (ie we/I basically have very little idea). In the photoelectric effect, though, it turns out that a metal seems to have a characteristic property of an energy which is needed to liberate it from the metal. Since WORK is done to liberate an electron from the metal, this energy is called the WORK FUNCTION. Millikan and the photoelectric effect.

5. AND if the photon energy coming in is equal to the work function energy (sitting there), then an electron could be ejected. If the photon energy is HIGHER than the work function energy, then an electron could be ejected AND have kinetic energy. Here, the total of the work function energy and the kinetic energy would equal the photon energy. So that, presumably, the higher the photon frequency, the “more energetic” the emitted photons whizzing about. Millikan and the photoelectric effect.

6. STOPPING POTENTIAL Millikan’s experiment is based around the idea of STOPPING the electrons which are emitted from the metal from going anywhere, especially towards his detectors. In this respect, this experiment, along with a host of others, could be thought of in UK A level physics terms as a “NULL EXPERIMENT”. That’s an experiment in which all the bits of the apparatus are arranged so that you get a “NULL READING”. An example would be a Wheatstone bridge, a slide wire potentiometer and maybe a myriad of others. Millikan’s experiment would use a potential on the metal to WORK against the KINETIC ENERGY of the electrons. And my brain’s just gone blank. Millikan and the photoelectric effect.

7. Millikan’s experiment would use a potential on the metal to WORK against the KINETIC ENERGY of the electrons. And my brain’s just gone blank. Presumably, he Professor Millikan didn’t fancy trying to measure the kinetic energy of the electrons directly. Perhaps, he thought, far easier, to STOP them, and measure the STOPPING ENERGY. And then relate this stopping energy to a stopping potential, or voltage, which he could measure using a voltmeter. At the stopping potential Millikan and the photoelectric effect.

9. Robert Andrews Millikan for his work on the elementary charge of electricity and on the photoelectric effectelementary chargephotoelectric effect Nobel prize awarded 1923 Millikan and the photoelectric effect.