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The photoelectric effect is the emission of electrons from a
surface (usually metallic) upon exposure to, and absorption of, electromagnetic radiation (such as visible light and
ultraviolet radiation) that is above the threshold frequency particular to each type of surface. No electrons are emitted for
radiation below the threshold frequency, as they cannot gain sufficient energy to overcome their atomic bonding.
The photoelectric effect helped further wave-particle
duality, whereby physical systems (such as photons in this case) can display both
wave-like and particle-like properties and behaviours, a concept that was used by the creators of quantum mechanics. The photoelectric effect was explained mathematically
by Albert Einstein utilising the work in quantum mechanics developed
by such people as Max Planck.
History
Hertz's Observations
The first recorded observation of the photoelectric effect was by Heinrich Hertz in 1887 when he was investigating the production and
reception of electromagnetic (EM) waves. His receiver consisted of a coil with a spark gap, whereupon a spark would be seen upon detection of EM waves. He placed the apparatus in a darkened box
in order to see the spark better; he observed, however, that the maximum spark length was reduced when in the box. The glass
panel placed between the source of EM waves and the receiver absorbed ultraviolet radiation that assisted the electrons in
jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he
substituted quartz for glass, as quartz does not absorb UV radiation.
Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this
effect, nor did he make any attempt at explaining how the observed phenomenon was brought about.
Electrons
In 1899, Joseph John
Thomson investigated ultraviolet light in cathode ray tubes. Influenced by the work of James Clerk Maxwell, Thomson
deduced that cathode rays consisted of negatively charged particles, which he called "corpuscles" (later called "electrons"). In
the research, Thomson enclosed a metal plate (i.e., cathode), in a vacuum tube, and exposed it to high frequency radiation. The
oscillating electromagnetic fields caused the atoms' field to be resonated and, after reaching a certain amplitude, caused by his
model a subatomic "corpuscle" to be emitted, and current to be detected. The current and speed of this current varied with the
intensity and color of the radiation. Larger increments of the radiation intensity or frequency of the field would produce more
current.
Radiant Energy
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US685957
Rays falling on insulated conductor connected to a capacitor: the capacitor charges electrically.
(larger)
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In 1901 on November 5, Nikola Tesla received the patent US685957 (Apparatus for the Utilization of Radiant Energy) that
describes radiation charging and discharging conductors (e.g., a metal plate or piece of
mica) by "radiant
energy". Tesla used this effect to charge a capacitor with energy by mean of a conductive plate (i.e., a solar cell precursor). The radiant energy threw off with great velocity minute
particles (i.e., electrons) which were strongly electrified. The patent specified that the radiation (or radiant energy) included
many different forms. These devices have been referred to as "Photoelectric alternating current stepping motors".
In practice, a polished metal plate in radiant energy (e.g. sunlight) will gain a positive charge as electrons are emitted by
the plate. As the plate charges positively, electrons form a electrostatic force on the plate (because of surface emissions of
the photoelectrons), and "drain" any negatively charged capacitors. As the rays or radiation fall on the insulated
conductor (which is connected to a capacitor), the condenser will indefinitely
charge electrically.
Light Quanta
Although the effect itself had been described earlier by Nikola Tesla in the patent US685957, Albert Einstein's theoretical model of the photoelectric effect helped him win the Nobel Prize of 1921. Einstein's mathematical
description in 1905 of how it was caused by absorption of photons, or quanta of light, in the interaction of
light with the electrons in the substance, was contained in the paper named "On
a Heuristic Viewpoint Concerning the Production and Transformation of Light". This paper proposed the simple description of
"light quanta" (later called "photons") and showed how they could be used to explain such phenomena as the photoelectric
effect. The simple explanation by Einstein in terms of absorption of single quanta of
light explained the features of the phenomenon and helped explain the characteristic frequency.
The idea of light quanta was motivated by Max Planck's published law of
black-body radiation ("On the Law of Distribution of
Energy in the Normal Spectrum". Annalen der Physik 4 (1901)) by assuming that Hertzian oscillators could only exist at
energies E proportional to the frequency f of the oscillator by E = hf where h is Planck's constant. Einstein, by assuming that
light actually consisted of discrete energy packets, wrote an equation for the photoelectric effect that fit
experiments. The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and,
more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that
Einstein's equations for the photoelectric effect were accurate, his explanation was not universally accepted.
The photons of the light beam have a characteristic energy given by the wavelength of the light. In the photoemission process, if an electron absorbs the energy of one photon and has
more energy than the work function, it is ejected from the material. If
the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of
the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted
electrons does not depend on the intensity of the incoming light. It was for this insight that Einstein won his only Nobel
Prize.
Explanations
The photoelectric effect helped propel the then-emerging concept of the dual nature of light (light exhibits characteristics of waves and particles at different times). It was difficult to understand in
terms of the classical wave description of light, as the energy of the emitted electrons
did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons could 'gather up' energy
over a period of time, and then be emitted. For such a classical theory to work a pre-loaded state would need to persist in
matter. The idea of the pre-loaded state was discussed in Millikan's book Electrons (+ & -) and in Compton and Allison's book
X-Rays in Theory and Experiment. These ideas were abandoned.
For a given surface, there is a minimum frequency (or maximum wavelength, since frequency and wavelength are inversely proportional), whereby
incident radiation with a lower frequency than the threshold, and hence lower energy than the minimum required, did not cause
electrons to be emitted, regardless of intensity.
Einstein explained this by imagining light as being broken into packets (quanta)
of energy. Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle. All of the
energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If
the photon is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's
kinetic (moving) energy as a free particle.
Philipp von Lenard observed the
following:
- The maximum stopping potential (voltage)in a phototube and hence the calculated maximum electron kinetic energy is determined by the frequency of the light. For example, an
increase in frequency results in a proportional increase in the maximum kinetic energy calculated for an electron upon liberation
- ultraviolet radiation would require a higher applied
stopping potential, to stop current in a phototube, than blue light.
- The current emitted by the surface is determined by the light's intensity, or brightness. For example, doubling the intensity
of the light doubles the number of electrons emitted from the surface. The number of electrons corresponds to the current
produced by a photocell.
Lenard did not know of photons. The experiment measures potentials, not electron kinetic energy. We calculate kinteic energy
from our model.
Equations
In analysing the photoelectric effect quantitatively using Einstein's method, the following equations are used:
Energy of photon = Energy needed to remove an electron + Kinetic energy of the emitted electron
Or
-
Or
- hf = φ + Ek
where h is Planck's constant,
f0 is threshold frequency for the photoelectric effect to
occur, φ is the work function, or minimum energy require to remove
electron from atomic binding, and Ek is maximum kinetic energy observed.
- Note: An electron will be emitted if hf > φ.
When this equation is not observed to be true, it may be because when given an excess amount of energy to the body, some
energy is absorbed as heat or emitted as radiation, as no system is perfectly efficient.
Common Uses
The photoelectric effect is used for such things as solar power by
mounting arrays of solar cells which generate direct current from sunlight, and light-sensitive diodes.
Electroscopes
Electroscopes are fork-shaped, hinged metallic leaves placed in a vacuum
jar, partially exposed to the outside environment. When an electroscope is charged positively or negatively, the two leaves
separate, as charge distributes evenly along the leaves causing repulsion between two like poles. When ultraviolet radiation (or
any radiation above threshold frequency) is shone onto the metallic outside of the electroscope, the negatively charged one will
discharge and collapse, while nothing will happen to the positively charged one. The reason is that electrons will be liberated
from the negatively charged one, gradually making it neutral, while liberating electrons from the positively charged one will
make it even more positive, keeping the leaves apart.
See also
External links and references
General
- Nave, R., "Wave-Particle Duality ". HyperPhysics.
- Jpaul's "Photovoltaics: Theory and Practice ". Photoelectric effect .
- "Photoelectric effect ". Physics 2000.
University of Colorado, Boulder, Colorado.
- ACEPT W3 Group, "The Photoelectric Effect ". Department of Physics and
Astronomy, Arizona State University, Tempe, AZ.
- Haberkern, Thomas, and N Deepak "Grains of Mystique: Quantum Physics for the Layman ". Einstein Demystifies Photoelectric Effect , Chapter 3.
- Department of Physics, "The Photoelectric effect ". Physics 320 Laboratory, Davidson
College, Davidson.
- Fowler, Michael, "The Photoelectric Effect ". Physics 252, University
of Virginia.
- Tesla, Nikola, "
Apparatus for the Utilization of Radiant Energy "
US685957. United States Patent and Trademark Office (USPTO), Issued Patents (PatFT).
- Beaty, Bill, "'Energy-sucking' Radio Antennas, N. Tesla's Power Receiver ".
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