MadSci Network: Physics

Re: What causes luminous watch dials to glow in the dark?

Date: Sun Nov 8 13:50:24 1998
Posted By: Bob Novak, Other (pls. specify below), Sr Process Research Engineer, Carpenter Technology
Area of science: Physics
ID: 909820742.Ph

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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