- 3356 - PHOTOELECTRIC EFFECT - the cause of strange behavior? When light interacted with electrons, light just didn't behave like it was supposed to. Repairing this tear in theory required more than just a patch. It meant rebuilding a large portion of physics from the ground up.
-- 3356 - PHOTOELECTRIC EFFECT - the cause of strange behavior?
- Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid. This process is called the photoelectric effect, and the ejected electrons are called “photoelectrons“; but there is nothing that would distinguish them from other electrons. All electrons are identical to one another in mass, charge, spin, and magnetic moment.
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- The photoelectric effect was first observed in 1887 by Heinrich Hertz during experiments with a spark gap generator (the earliest device that could be called a radio). In these experiments, sparks generated between two small metal spheres in a transmitter induce sparks that jump between two different metal spheres in a receiver.
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- Compared to later radio devices, the spark gap generator was notoriously difficult to work with. The air gap would often have to be smaller than a millimeter for a the receiver to reliably reproduce the spark of the transmitter. Hertz found that he could increase the sensitivity of his spark gap device by illuminating it with visible or ultraviolet light.
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- Later studies by J. J. Thomson showed that this increased sensitivity was the result of light pushing on electrons, a particle that he discovered in 1897.
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- Subsequent investigations into the photoelectric effect yielded results that did not fit with the classical theory of electromagnetic radiation. When it interacted with electrons, light just didn't behave like it was supposed to. Repairing this tear in theory required more than just a patch. It meant rebuilding a large portion of physics from the ground up.
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- It was Philipp Lenard, an assistant of Hertz, who performed the earliest, definitive studies of the photoelectric effect. Lenard used metal surfaces that were first cleaned and then held under a vacuum so that the effect might be studied on the metal alone and not be affected by any surface contaminants or oxidation.
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- The metal sample was housed in an evacuated glass tube with a second metal plate mounted at the opposite end. The tube was then positioned or constrained in some manner so that light would only shine on the first metal plate, the one made out of photoemissive material under investigation.
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- Such a tube is called a “photocell” or an “electric eye“. Lenard connected his photocell to a circuit with a variable power supply, voltmeter, and microammeter. He then illuminated the photo-emissive surface with light of differing frequencies and intensities.
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- Photocell, voltmeter, and variable power supply in parallel. Micro-ammeter in series.
Knocking electrons free from the photoemissive plate would give it a slight positive charge. Since the second plate was connected to the first by the wiring of the circuit, it too would become positive, which would then attract the photoelectrons floating freely through the vacuum where they would land and return back to the plate from which they started.
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- This experiment doesn't create electrons out of light, it just uses the energy in light to push electrons that are already there around the circuit. The photoelectric current generated by this means was quite small.
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- The small photoelectric current could be measured with the micro-ammeter (a sensitive galvanometer with a maximum deflection of only a few microamps). It also serves as a measure of the rate at which photoelectrons are leaving the surface of the photoemissive material.
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- What Lenard found was that the intensity of the incident light had no effect on the maximum kinetic energy of the photoelectrons. Those ejected from exposure to a very bright light had the same energy as those ejected from exposure to a very dim light of the same frequency. In keeping with the law of conservation of energy, however, more electrons were ejected by a bright source than a dim source.
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- Later experiments by others, most notably the American physicist Robert Millikan in 1914, found that light with frequencies below a certain cutoff value, called the “threshold frequency“, would not eject photoelectrons from the metal surface no matter how bright the source was.
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- These result were completely unexpected. Given that it is possible to move electrons with light and given that the energy in a beam of light is related to its intensity, classical physics would predict that a more intense beam of light would eject electrons with greater energy than a less intense beam no matter what the frequency. This was not the case, however.
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------------------------ Red light does not eject photoelectrons (even if it is very bright).
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------------------------ Green light does eject photoelectrons (even if it is very dim).
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------------------------ Blue light ejects photoelectrons with more energy than green light (even if it is very dim).
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- Actually, these results aren't all that typical. Most elements have threshold frequencies that are ultraviolet and only a few dip down low enough to be green or yellow like the example shown above. The materials with the lowest threshold frequencies are all semiconductors. Some have threshold frequencies in the infrared region of the spectrum.
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- The classical model of light describes it as a transverse, electromagnetic wave. Of this there was very little doubt at the end of the 19th century. The wave nature of light was confirmed when it was applied successfully to explain such optical phenomena as diffraction, interference, polarization, reflection and refraction.
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- If we can imagine light as waves in an electromagnetic ocean and be quite successful at it, then it wouldn't be much of a stretch for us to image electrons in a metal surface as something like tethered buoys floating in an electromagnetic harbor. Along come the waves (light) which pull and tug at the buoys (electrons). Weak waves have no effect, but strong ones just might yank a buoy from their mooring and set it adrift.
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- A wave model of light would predict an energy-amplitude relationship and not the energy-frequency relationship described above. Photoelectric experiments describe an electromagnetic ocean where monstrous swells wouldn't tip over a canoe, but tiny ripples would fling you into the air.
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- If that wasn't enough, the photoelectrons seem to pop out of the surface too quickly. When light intensities are very low, the rate at which energy is delivered to the surface is downright sluggish. It should take a while for any one particular electron to capture enough of this diffuse energy to free itself. It should, but it doesn't.
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- The instant that light with an appropriate frequency of any intensity strikes a photoemissive surface, at least one electron will always pop out immediately
(time < 10^−9 seconds).
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- Continuing with the ocean analogy, imagine a harbor full of small boats (electrons). The sea is calm except for tiny ripples on the surface (low intensity, short wavelength light). Most of the boats in the harbor are unaffected by these waves, but one is ripped from the harbor and sent flying upward like a jet aircraft. Something just ain't right here. No mechanical waves behave like this, but light does.
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- The two factors affecting maximum kinetic energy of photoelectrons are the frequency of the incident radiation and the material on the surface. Electron energy increases with frequency in a simple linear manner above the threshold.
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- All three curves have the same slope (equal to Planck's constant) which shows that the energy-frequency relation is constant for all materials. Below the threshold frequency photoemission does not occur. Each curve has a different intercept on the energy axis, which shows that threshold frequency is a function of the material.
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- The genius that figured out what was going on here was none other than the world's most famous physicist Albert Einstein. In 1905, Einstein realized that light was behaving as if it was composed of tiny particles (initially called quanta and later called photons) and that the energy of each particle was proportional to the frequency of the electromagnetic radiation that it was a part of.
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- Max Planck invented the notion of quantized electromagnetic radiation as a way to solve a technical problem with sources of electromagnetic radiation called blackbodies.
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- Planck did not believe that radiation was actually broken up into little bits as his mathematical analysis showed. He thought the whole thing was just a “contrivance” that gave him the right answers.
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- The genius of Einstein was in recognizing that Planck's contrivance was in fact a reasonable description of reality. What we perceive as a continuous wave of electromagnetic radiation is actually a stream of discrete particles.
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- In fact, the observations on "black-body radiation", photo-luminescence, the production of cathode rays by ultraviolet light and other phenomena involving the emission or conversion of light can be better understood on the assumption that the energy of light is distributed discontinuously in space.
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- According to the assumption considered here, when a light ray starting from a point is propagated, the energy is not continuously distributed over an ever increasing volume, but it consists of a finite number of energy quanta, localized in space, which move without being divided and which can be absorbed or emitted only as a whole.
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- Einstein and Millikan described the photoelectric effect using a formula that relates the maximum kinetic energy (Kmax) of the photoelectrons to the frequency of the absorbed photons (f) and the threshold frequency (f0) of the photoemissive surface:
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----------------------------- Kmax = h(f − f0)
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- or, to the energy of the absorbed photons (E) and the work function (φ) of the surface
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----------------------------- Kmax = E − φ
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-where the first term is the energy of the absorbed photons (E) with frequency (f) or wavelength (λ)
----------------------------- E = h * f = hc * λ
and the second term is the work function (φ) of the surface with threshold frequency (f0) or threshold wavelength (λ0)
----------------------------- φ = h * f0 = hc * λ0
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The maximum kinetic energy (Kmax) of the photoelectrons (with charge e) can be determined from the stopping potential (V0).
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----------------------------- V0 = W = Kmax q e
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----------------------------- Kmax = e * V0
When charge (e) is given in coulombs, the energy will be calculated in joules. When charge (e) is given in elementary charges, the energy will be calculated in electron volts.
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------------------------------- Planck's constant with variations
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----------------------------- h = 6.63 × 10^−34 Js = 4.14 × 10^−15 eV s
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----------------------------- hc = 1.99 × 10^−25 Jm = 1240 eV nm
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- The rate (n/t) at which photoelectrons (with charge e) are emitted from a photoemissive surface can be determined from the photoelectric current (I).
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----------------------------- I = q = ne / t
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----------------------------- n = I * t / e
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---------------------------- photoconductivity: an increase in the electrical conductivity of a nonmetallic solid when exposed the electromagnetic radiation. The increase in conductivity is due to the addition of free electrons liberated by collision with photons. The rate at which free electrons are generated and the time they over which the remain free determines the amount of the increase.
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----------------------------- photovoltaics: the ejected electron travels through the emitting material to enter a solid electrode in contact with the photoemitter (instead of traveling through a vacuum to an anode) leading to the direct conversion of radiant energy to electrical energy, photostatic copying
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- Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid. This process is called the photoelectric effect (or photoelectric emission or photoemission), a material that can exhibit this phenomena is said to be photoemissive, and the ejected electrons are called photoelectrons; but there is nothing that would distinguish them from other electrons.
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-------------------------- All electrons are identical to one another in mass, charge, spin, and magnetic moment.
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- When light interacted with electrons, light just didn't behave like it was supposed to. Repairing this tear in theory required more than just a patch. It meant rebuilding a large portion of physics from the ground up.
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November 28, 2021 PHOTOELECTRIC EFFECT - strange behavior? 3356
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