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photoelectric effect (1 Viewer)
- Thread starter zinc
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Rachaek
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- 2008
With the photoelectric effect they found that only light above a specific frequency (threshold frequency) would eject electrons from the metal, regardless of the intensity of the light or the time the metal was exposed.
The wave model predicted that if the metal was exposed to light for a long period of time, the electrons would eventually accumulate enough energy to escape from the metal, however this was not the case. They also predicted that a higher intensity would 'make up for' the lower frequency, and electrons would still be ejected, but again this was not found.
Only was to explain it is through photons/quantum ideas. Electrons can't gain energy incrementally, they either absorb a whole photon of hight enough energy and are ejected, or don't absorb one at all. So they are only ejected when the energy of the photons (frequency of the light) is high enough. The intensity corresponds to the number of photons.
Hope that helped.
The wave model predicted that if the metal was exposed to light for a long period of time, the electrons would eventually accumulate enough energy to escape from the metal, however this was not the case. They also predicted that a higher intensity would 'make up for' the lower frequency, and electrons would still be ejected, but again this was not found.
Only was to explain it is through photons/quantum ideas. Electrons can't gain energy incrementally, they either absorb a whole photon of hight enough energy and are ejected, or don't absorb one at all. So they are only ejected when the energy of the photons (frequency of the light) is high enough. The intensity corresponds to the number of photons.
Hope that helped.
A few reasons.
Classically:
1) For a wave, increasing the intensity increases the energy of the system. E=1/2kA^2
2) There should be NO relationship between Kinetic energy and the frequency of light used
3) There should be a timelag between light striking the surface and electron being ejected (as the electron "builds up" energy to become ejected
These are all predictions of Classical wave theory.
Quantum Mechanically:
1) Einstein proposed that light consists of particles called photons each with Energy E = hf.
According to this theory as the frequency increases, Electrons are ejected with more Kinetic energy. Provided the photons have a high enough frequency they will eject the electrons. (technically they get absorbed so we should call them photoelectrons)
2) Intensity = Nchf where N = number of photons c = speed of light hf= Energy of a photon.
Thus increasing the intensity, increases the NUMBER of photons (which increases the current) however has NO EFFECT ON KINETIC ENERGY
3) Energy is delivered in a collision, thus all the energy is transferrred at once, this explains why there is NO time lag.
Classically:
1) For a wave, increasing the intensity increases the energy of the system. E=1/2kA^2
2) There should be NO relationship between Kinetic energy and the frequency of light used
3) There should be a timelag between light striking the surface and electron being ejected (as the electron "builds up" energy to become ejected
These are all predictions of Classical wave theory.
Quantum Mechanically:
1) Einstein proposed that light consists of particles called photons each with Energy E = hf.
According to this theory as the frequency increases, Electrons are ejected with more Kinetic energy. Provided the photons have a high enough frequency they will eject the electrons. (technically they get absorbed so we should call them photoelectrons)
2) Intensity = Nchf where N = number of photons c = speed of light hf= Energy of a photon.
Thus increasing the intensity, increases the NUMBER of photons (which increases the current) however has NO EFFECT ON KINETIC ENERGY
3) Energy is delivered in a collision, thus all the energy is transferrred at once, this explains why there is NO time lag.
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