|MadSci Network: Physics|
Hi Frank, First a definition, then the watch question. Luminescence is a low energy process for the emission of light. Incandescence is a high-energy process for emission of light. Historically, the first glow in the dark watches used radium paint. The radium decays by emission of an alpha particle. An alpha particle has 2 protons and 2 neutrons with no electrons. The emission of the alpha particle alone does not produce visible light. The light is produced when the radium is mixed with something that interacts with the alpha particle and emits visible light. Zinc sulfide was commonly mixed with radium to produce luminescent paint. Alpha radiation is dangerous and the use of radium in luminescent paints was discontinued after workers began dying from the high exposure that resulted from hand painting watch dials. Watch dials that can be viewed in the dark had become popular with consumers and the watch companies needed to find alternatives. The simplest way to illuminate a watch dial used a battery and a light bulb. Incandescent light bulbs are relatively inefficient; they produce more heat than light. The typical watch dial was only illuminated when a button was depressed to conserve battery life. More recently, electroluminescence is used in glow in the dark watches. A small electrical current excites the electrons in the luminescent material. When the electron returns to a lower energy state it emits a photon of light. The quantity of light produced (lumens) is not large, but it is enough for illuminating watch dials at night with low energy demand on the watch battery. Niels Bohr, a Danish physicist, observed that electrons in an atom have discrete energy levels or quanta. These energy steps are often equal in energy to a photon of visible light. In the electroluminescent material the electron is excited, gains energy, from an electric current. It looses energy through the emission of a photon of visible light. Quantum theory describes all matter as occupying discrete energy levels. This simplified description of quantum theory is relatively straightforward and easy to accept. One of the stranger parts of quantum theory states that the exact energy content of a particle can never be precisely determined. In quantum theory, only a statistical probability of a particle being in a given energy state can be determined. Statistically a particle has some probability, even though the probability might be very small, of being in any of its quantized energy states. So where is all of this quantum stuff going and how does it relate to the second part of your question? Alpha particles and “tunneling” - The energy source in radium based luminescent paint is the radioactive decay of an alpha particle. Radioactive isotopes of elements decay because they have an unstable number of protons and neutrons. The radioactive nucleus is in a higher energy state. The decay process reduces the energy of the nucleus. There is an energy barrier that must be overcome for the nucleus to decay. If the barrier were not present, all radioactive isotopes would instantly decay. So why do isotopes decay? Quantum theory is used to describe nuclear processes. Recall that in quantum theory the nucleus has a probability of occupying any permitted energy state. The transition from a higher to a lower energy state, decay of the nucleus, occurs by a process known as tunneling. To an observer, the nucleus appears to have tunneled under the energy barrier. Quantum theory says that nothing different has occurred, the nucleus is just occupying a different and permitted energy state. As strange as quantum theory might seam to the ordinary person, it does explain most of the observations of the physical world around us. Activation by ordinary light - Another method for producing a luminescent dial uses fluorescence. In fluorescence the electrons in an atom absorb a photon of light. The electrons are excited into higher energy levels by absorption of a photon of light (there is that quantum theory again). When the electron returns to a lower energy state, the energy is released as a photon of light. The fluorescent photon is lower in energy, longer in wavelength, than the photon that was originally absorbed. That is how exposing it to light activates a fluorescent watch dial. The incoming electromagnetic radiation (light) is absorbed by the electron, but only when the wavelength (= energy) exactly matches the energy needed to excite the electron. Ordinary light (visible wavelengths) cannot effect the decay rate of radioactive nuclei. I hope that answered all of your questions, Bob Novak
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