A B C
D
D. Three quantum phenomena
In fluorescence, matter absorbs light waves of a high frequency and then
emits light of the same or lower frequency. This process was studied and
named by George Gabriel Stokes in the mid-19th century. Today,
fluorescence is familiar to us from fluorescent light bulbs. A fluorescent
bulb's filament produces ultraviolet light, which is absorbed by the bulb's
inner coating, which then emits lower-frequency visible light-more visible
light than an incandescent bulb produces with the same wattage.
According to the hypothesis of light quanta, during fluorescence an atom
absorbs a quantum of light whose energy is proportional to the light wave's
frequency. If the atom doesn't supply any extra energy of its own, the
light quantum emitted should either have the same energy or less energy than the
quantum it absorbs. This means that the emitted light's frequency is
either the same or lower. Einstein also saw that, if light quanta really
existed, there might be deviations from this process. If we exposed an
atom to so many quanta at once that it could readily absorb more than one at a
time, the atom might combine the energies of the quanta to emit one single
quantum of higher frequency. Deviations might also happen if either the
absorbed or the emitted light had a low enough frequency, since Einstein's
analysis had begun by assuming the light waves' frequencies were high enough for
Figure 3's lower curve to accurately describe their intensity.
Another of Einstein's deductions relates to the way ultraviolet light can
generate positive and negative electric charges when it shines through
gases. If light moved as individual quanta, this process also ought to
work a certain way; positive and negative charges within a gas molecule would
separate once the molecule absorbed a sufficiently energetic quantum of
ultraviolet light. The energy would have to be enough for the charges to
completely overcome their mutual attraction and move apart. Since some
minimum amount of energy would be required, the light wave would have to have at
least a minimum frequency. Furthermore, since the energy to separate
positive and negative charges in one molecule would require one light quantum,
the amount of light required to separate charges in a given amount of gas should
be directly proportional to the amount of gas. Einstein noted that
checking this proportionality with experiments would be an important test of the
light-quantum hypothesis.
Einstein also described one other deduction from the quantum hypothesis
related to the photoelectric effect by which light shining on the surface of a
metal produces electrons. This deduction was especially significant since
others had already found certain features of this effect hard to explain if the
energy of light waves were distributed uniformly.
The electrons of a nonmetallic material are bound within atoms of the
material. In a metal, some of the electrons are less tightly bound, and
are free to move around throughout the volume of the metal instead of remaining
within a particular atom. However, these electrons are still bound to the
rest of the metal as a whole. They could escape from the metal if they had
enough energy to overcome its attraction. Shining a light on the metal can
supply extra energy.
It turns out that the electrons will only acquire enough energy if the light
shining on them has at least a minimum frequency. Maxwellian theory offers
no obvious explanation for this, but it is easy to explain in terms of light
quanta. Suppose each light quantum is absorbed by one electron.
Since the energy of a light quantum is proportional to its frequency, the
electron will have enough energy to break free of the metal if the frequency of
the quantum is high enough. If the frequency of the quantum is too low,
the electron won't gain enough energy to escape.
The required minimum frequency of the light waves can be raised by increasing
the metal's attraction to the electrons. Since electrons have a negative
charge, we can attract them more strongly by applying extra positive charge to
the metal. Einstein pointed out that certain conclusions follow from this
fact, at least if the light has high enough frequencies for lower curve in Figure
3 to be accurate. If each quantum transfers its energy to electrons
independently of all other quanta, the positive charge required to stop
electrons from leaving the metal should not depend on the intensity of the
light, while the number of electrons that would leave if the metal had a lower
voltage should increase at a fixed rate with the intensity of the light.
These predictions were verified in detail in experiments performed about a
decade later by Robert Millikan and his students. But the underlying ideas
about light quanta gained acceptance more slowly among physicists.
Today, Einstein is most famous for relativity. But of all the papers
Einstein was publishing in 1905, it was his paper on light quanta that he
regarded as the revolutionary one. For while his special relativity theory
revealed a surprising connection between space and time, that connection could
be logically deduced from known facts and existing physical theory. There
was no obvious way to account for light quanta in Maxwell's theory.
Planck had understood light quanta to be units of energy, but not as having
particular locations in space; Einstein brought the latter concept to
light. Einstein's predictions about the photoelectric effect were widely
accepted among physicists after Millikan and his students verified them about a
decade later, but it would be about one more decade before the concept behind
those predictions became similarly accepted. By then, further extensions
of the light-quantum concept into related ideas about matter had demonstrated
their value for understanding the behavior of the atoms and subatomic particles
of which matter is made. The value of these ideas, known as quantum
theory, is much clearer today, since atoms and particles are increasingly
the focus of practical technologies.
While the Nobel Prize awarded to Einstein in 1922 implicitly recognized his
now-famous theory of relativity, it specifically mentioned only his
accomplishment in describing how energies from photoelectrons depend on light
frequency, and how the number of photoelectrons depends on light
intensity. The presentation speech did describe Einstein's work on
Brownian motion and the relativity theory (pointing out how, at that time, some
philosophers challenged this theory while others acclaimed it), but Einstein's
photoelectric theory was the main topic. In fact, the award diploma
declared the prize was awarded to Einstein "for his services to Theoretical
Physics, and especially for his discovery of the law of the photoelectric
effect".
Next article:
Solid Cold
References, Links, and Comments:
"The Nobel Prize in Physics 1921" [exit federal site]
[exit federal site]The 1921 Nobel Prize in Physics was actually awarded a year later, at the
same ceremony where Niels Bohr received the Physics prize for 1922.
The presentation speech describes the law of the photoelectric effect and
Einstein's concept of how the effect works.
"
The Photoelectric Effect"
[exit federal site] by Michael Fowler
A brief history of the effect's discovery and the experimental analysis of
its details.
"The Photoelectric Effect" [exit federal site] by Walter Fendt
A Java applet simulating a Millikan-type experiment.
The Old Quantum Theory, edited by D.
ter Haar
Contains English translations of the December 1900 paper by Max Planck cited
above ("On the Theory of the Energy Distribution Law of the Normal
Spectrum") and of the paper in which Albert Einstein published the discoveries
described in this article ("On a Heuristic Point of View about the Creation
and Conversion of Light").
"A Scientific Autobiography" (English translation of "Wissenschaftliche
Selbstbiographie"), in Scientific
Autobiography and Other Papers by Max Planck.
Among other things, describes how and why Planck worked out the relation between
intensity and frequency of the cavity radiation described above, also known as black-body
radiation.
The Rise of the New Physics by Abraham
D'Abro
The section "The Wave Theory of Light" (pp. 276-281) describes the theory
proposed by Christiaan Huygens in the 17th century and its later
development by others, as well as experiments by Huygens and later physicists
demonstrating light's wave character. The
chapter "Planck's Original Quantum Theory" (pp. 447-471) is a detailed
discussion of Planck's theory of cavity radiation and its predecessors; the
section "Rayleigh's Erroneous Law of Radiation" (pp. 454-456) tells why
the older theory of light leads to the erroneous upper curve in Figure
2.
Treatise
on Light [exit federal site] by Christiaan Huygens (Silvanus P. Thompson, trans.)
"The 8 January 1690".
Chapter V, "On
the Strange Refraction of Iceland Crystal", [exit federal site] contains a section [exit federal site]
describing a polarization phenomenon
perpendicular to the way light travels through the crystal-which was later
understood to be the direction of the light's vibrating force fields.
"George
Gabriel Stokes", [exit federal site] The
MacTutor History of Mathematics [exit federal site] archive, School
of Mathematics and Statistics, [exit federal site] University
of St Andrews, Scotland
[exit federal site]
Short biography of the discoverer of fluorescence.
The Strange Story of the Quantum
by Banesh Hoffmann
Heinrich Hertz' experiments with electromagnetic waves are described early in
this book. Interestingly, as this
book shows, it was Hertz, during this demonstration of the correctness of Maxwell's theory, who discovered the photoelectric effect, which
presented a new problem for that same theory.
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Prepared by Dr. William Watson, Physicist
DOE Office of Scientific and Technical Information