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Pendleton College AS/A2 Physics Forum :: A2 Revision Unit 5 Optional Topics :: Discovery of Photoelectricity
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Photoelectric Effect
jmorris Sat Apr 21, 2012 1:48 am
Photoelectric Effect occurs when light is incident on a metal surface causing electrons to be emitted.
Photoelectric emission was first noticed by Hertz when he was investigating radio waves.
Further investigations showed that:
No photoelectrons were emitted if the frequency of the incident light was below a certain value. This is the threshold frequency. This can be explained by treating light as quanta of electromagnetic radiation (packets). Each would have energy, E=hf. As the frequency increases the energy of the photons increases. 1 photon transfers all its energy to 1 electron. If the photon energy is greater than or equal to the work function of the metal, then electrons are emitted. This is instantaneous. Any energy the photon has above the work function of the metal becomes kinetic energy of the photoelectrons.
The photoelectrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value. The kinetic energy is not affected by the intensity of the incident light as the photon energy depends only on frequency.
The number of photons, therefore number of photoelectrons emitted per second is directly proportional to the intensity of the radiation.
This cannot be explained by wave theory. Wave theory would predict that light of all frequencies would cause photoelectrons to be emitted, it would just take time for enough energy to be delivered to each electron.
Also energy is proportional to intensity, energy carried by the light would be spread evenly across the wavefront and each electron on the surface of the metal would get a bit of energy from each incoming wave.
Photoelectric emission was first noticed by Hertz when he was investigating radio waves.
Further investigations showed that:
No photoelectrons were emitted if the frequency of the incident light was below a certain value. This is the threshold frequency. This can be explained by treating light as quanta of electromagnetic radiation (packets). Each would have energy, E=hf. As the frequency increases the energy of the photons increases. 1 photon transfers all its energy to 1 electron. If the photon energy is greater than or equal to the work function of the metal, then electrons are emitted. This is instantaneous. Any energy the photon has above the work function of the metal becomes kinetic energy of the photoelectrons.
The photoelectrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value. The kinetic energy is not affected by the intensity of the incident light as the photon energy depends only on frequency.
The number of photons, therefore number of photoelectrons emitted per second is directly proportional to the intensity of the radiation.
This cannot be explained by wave theory. Wave theory would predict that light of all frequencies would cause photoelectrons to be emitted, it would just take time for enough energy to be delivered to each electron.
Also energy is proportional to intensity, energy carried by the light would be spread evenly across the wavefront and each electron on the surface of the metal would get a bit of energy from each incoming wave.
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Join date : 2012-01-31
Pendleton College AS/A2 Physics Forum :: A2 Revision Unit 5 Optional Topics :: Discovery of Photoelectricity
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