24 june2017. Learning physics basics. photoelectric effect.
Einstein's explanation of the photoelectric effect
The photoelectric effect. Incoming photons on the left strike a metal
plate (bottom), and eject electrons, depicted as flying off to the
right.
In the photoelectric effect, it was observed that shining a light on certain metals would lead to an electric current in a circuit. Presumably, the light was knocking electrons out of the metal, causing current to flow. However, using the case of potassium as an example, it was also observed that while a dim blue light was enough to cause a current, even the strongest, brightest red light available with the technology of the time caused no current at all. According to the classical theory of light and matter, the strength or amplitude of a light wave was in proportion to its brightness: a bright light should have been easily strong enough to create a large current.
Yet, oddly, this was not so.
Einstein explained this enigma by postulating that the electrons can receive energy from electromagnetic field only in discrete portions (quanta that were called photons): an amount of energy E that was related to the frequency f of the light by
Only photons of a high enough frequency (above a certain threshold value) could knock an electron free.
For example, photons of blue light had sufficient energy to free an electron from the metal, but photons of red light did not.
One photon of light above the threshold frequency could release only one electron; the higher the frequency of a photon, the higher the kinetic energy of the emitted electron, but no amount of light (using technology available at the time) below the threshold frequency could release an electron.
To "violate" this law would require extremely high-intensity lasers which had not yet been invented.
Intensity-dependent phenomena have now been studied in detail with such lasers.[14]
Einstein was awarded the Nobel Prize in Physics in 1921 for his discovery of the law of the photoelectric effect.
ReplyDeleteEinstein explained this enigma by postulating that the electrons can receive energy from electromagnetic field only in discrete portions (quanta that were called photons): an amount of energy E that was related to the frequency f of the light by
E=hf\,
where h is Planck's constant (6.626 × 10−34 J seconds).
Only photons of a high enough frequency (above a certain threshold value) could knock an electron free.
For example, photons of blue light had sufficient energy to free an electron from the metal, but photons of red light did not.