Einstein Centennial -- 2005
Einstein's first significant paper, on the photoelectric effect, was sent to the journal Annalen der Physik on March 17, 1905, three days after his 26th birthday.
In the photoelectric effect experiment, shining light on a metal plate can cause the ejection of electrons from the metal plate. To measure the properties of the ejected electrons, one introduces a second metal plate, and applies a voltage differential across the two plates. If one starts with zero voltage across the plates, then some current will flow when the light is turned on: electrons knocked off the "source" plate with any speed will eventually reach the "target" plate. But if one gradually increases the voltage difference (to impede electron transit) fewer and fewer electrons will make the journey, and, at some voltage, the current disappears. The voltage wherein the current disappears can be used to give a measure of the maxiumum kinetic energy of the ejected electron.
Experimental observations in the photoelectric effect are:
--> The energy of the electrons does NOT depend on the intensity of the light.
--> The electrons always appear AS SOON AS the light reaches the plate (though a feeble light produces only a few).
--> NO electrons are produced if the frequency of the light waves is below a critical value.
That is, as the brightness (intensity) of light incident on a metal surface increased, more electrons were ejected from the metal. However, the energy of the electrons depended only on the frequency of the light, not on the intensity of the light. Shining more intense light (even for example a laser light) would not increase the energy of the electrons. To explain this, Einstein proposed energy quanta, which both explained the photoelectric effect and avoided the so-called ultraviolet catastrophe.
-->The energy of the electrons does NOT depend on the intensity of the light.
Each electron absorbs only one photon at a time. If the absorbed energy is large enough to expel the electron from the metal, it leaves. If not, the electron dissipates its energy in collisions with nearby electrons and atoms before it can absorb another photon.
-->The electrons always appear AS SOON AS the light reaches the plate (though a feeble light produces only a few).
As soon as a single photon containing sufficient energy strikes the source plate, it will knock an electron free. There is no need to wait for multiple waves to build up enough energy.
-->NO electrons are produced if the frequency of the light waves is below a critical value (no matter what the intensity of the light is)
Since the energy of each photon is
E = hv
below some critical frequency v, no photon has enough energy to knock an electron free.
Moreover, Einstein's theory was able to make one very strong prediction: the maximum energy of ejected electrons should increase linearly with frequency of the applied light.
On May 11, 1905, Annalen received Einstein's paper on Brownian motion, and on June 30, 1905, the paper on special relativity. Thus, three significant papers were sent out in a little over three months. Einstein at the time was working at the Swiss Patent Office in Berne.
**
As a minor point, in the play Camping with Henry and Tom (by Mark St. Germain, nominally based in time around 1921), Thomas Edison mentions that he does not understand the photoelectric effect. However, Edison in 1883 discovered the "Edison effect" wherein electric current would flow from a light bulb filament to a positively charged metal plate inside the light bulb. Edison obtained a patent on using the "Edison effect" as a current measuring device; the flow of current was proportional to the incandescence of the bulb. More importantly, the "Edison effect" is the basis for the vacuum tube developed by Fleming and DeForest.
Fleming (who at one time worked for Edison) noted that the alternating current being applied to the bulb filament was leaving the tube (via the metal plate) as direct current. Such tube was called the Fleming Valve and its ability to rectify AC current was used in the Diode tubes that followed (and the same concept applies to silicon diodes today). DeForest recognized the opportunity for amplification and modified the tube so it could not only rectify AC current but also amplify it. The De Forest Audion (now triode) contained the filament and plate of the Fleming valve, but interposed between them was a zigzag wire called the grid. A small electric current applied to the grid would result in the proportionate flow of a much larger current from the filament to the plate- creating amplification.
The field effect transistor (FET) was developed by analogy to the diode and triode:
FET/(solid state rectifier) : triode/diode.
This analogy was recognized (and patented by) Lilienfeld approximately twenty years BEFORE Bell Labs developed the transistor.
In the photoelectric effect experiment, shining light on a metal plate can cause the ejection of electrons from the metal plate. To measure the properties of the ejected electrons, one introduces a second metal plate, and applies a voltage differential across the two plates. If one starts with zero voltage across the plates, then some current will flow when the light is turned on: electrons knocked off the "source" plate with any speed will eventually reach the "target" plate. But if one gradually increases the voltage difference (to impede electron transit) fewer and fewer electrons will make the journey, and, at some voltage, the current disappears. The voltage wherein the current disappears can be used to give a measure of the maxiumum kinetic energy of the ejected electron.
Experimental observations in the photoelectric effect are:
--> The energy of the electrons does NOT depend on the intensity of the light.
--> The electrons always appear AS SOON AS the light reaches the plate (though a feeble light produces only a few).
--> NO electrons are produced if the frequency of the light waves is below a critical value.
That is, as the brightness (intensity) of light incident on a metal surface increased, more electrons were ejected from the metal. However, the energy of the electrons depended only on the frequency of the light, not on the intensity of the light. Shining more intense light (even for example a laser light) would not increase the energy of the electrons. To explain this, Einstein proposed energy quanta, which both explained the photoelectric effect and avoided the so-called ultraviolet catastrophe.
-->The energy of the electrons does NOT depend on the intensity of the light.
Each electron absorbs only one photon at a time. If the absorbed energy is large enough to expel the electron from the metal, it leaves. If not, the electron dissipates its energy in collisions with nearby electrons and atoms before it can absorb another photon.
-->The electrons always appear AS SOON AS the light reaches the plate (though a feeble light produces only a few).
As soon as a single photon containing sufficient energy strikes the source plate, it will knock an electron free. There is no need to wait for multiple waves to build up enough energy.
-->NO electrons are produced if the frequency of the light waves is below a critical value (no matter what the intensity of the light is)
Since the energy of each photon is
E = hv
below some critical frequency v, no photon has enough energy to knock an electron free.
Moreover, Einstein's theory was able to make one very strong prediction: the maximum energy of ejected electrons should increase linearly with frequency of the applied light.
On May 11, 1905, Annalen received Einstein's paper on Brownian motion, and on June 30, 1905, the paper on special relativity. Thus, three significant papers were sent out in a little over three months. Einstein at the time was working at the Swiss Patent Office in Berne.
**
As a minor point, in the play Camping with Henry and Tom (by Mark St. Germain, nominally based in time around 1921), Thomas Edison mentions that he does not understand the photoelectric effect. However, Edison in 1883 discovered the "Edison effect" wherein electric current would flow from a light bulb filament to a positively charged metal plate inside the light bulb. Edison obtained a patent on using the "Edison effect" as a current measuring device; the flow of current was proportional to the incandescence of the bulb. More importantly, the "Edison effect" is the basis for the vacuum tube developed by Fleming and DeForest.
Fleming (who at one time worked for Edison) noted that the alternating current being applied to the bulb filament was leaving the tube (via the metal plate) as direct current. Such tube was called the Fleming Valve and its ability to rectify AC current was used in the Diode tubes that followed (and the same concept applies to silicon diodes today). DeForest recognized the opportunity for amplification and modified the tube so it could not only rectify AC current but also amplify it. The De Forest Audion (now triode) contained the filament and plate of the Fleming valve, but interposed between them was a zigzag wire called the grid. A small electric current applied to the grid would result in the proportionate flow of a much larger current from the filament to the plate- creating amplification.
The field effect transistor (FET) was developed by analogy to the diode and triode:
FET/(solid state rectifier) : triode/diode.
This analogy was recognized (and patented by) Lilienfeld approximately twenty years BEFORE Bell Labs developed the transistor.
posted by Lawrence B. Ebert at 2:26 PM
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