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Question
What is the photoelectric effect? State the result of a photoelectric effect experiment that could not be explained on the basis of laws of classical physics. Explain this effect on the basis of quantum theory of electromagnetic radiations.
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Solution
Understanding the photoelectric effect.
Photoelectric effect states that when specific metals are exposed to a sufficiently energetic light beam, the metal ejects the electrons.
This phenomenon is known as the photoelectric effect. The electrons ejected are called photoelectrons.
Explaining the result of the photoelectric effect.
i) The electrons are ejected from the metal surface as soon as the beam of light strikes the surface.
ii) The number of electrons ejected is proportional to the intensity of radiation.
iii) For each metal; there is a minimum frequency, below which photoelectric effect is not observed. This frequency is known as threshold frequency.
iv) Kinetic energy of electrons is proportional to the frequency of light.
Explanation of the photoelectric effect on the basis of quantum theory
Einstein was able to explain the photoelectric effect using Planck’s quantum theory of electromagnetic radiation as a starting point
Shining a beam of light on to a metal surface can, therefore, be viewed as shooting a beam of particles, the photons. When a photon of sufficient energy strikes an electron in the atom of the metal, it transfers its energy instantaneously to the electron during the collision and the electron is ejected without any time lag or delay.
Greater the energy possessed by the photon, greater will be transfer of energy to the electron and greater the kinetic energy of the ejected electron. So, considering photon as particles on basis of quantum theory does explain the results of photoelectric effect.
Since the striking photon has energy equal to hν and the minimum energy required to eject the electron is hν0(also called work function, W0),
then the difference in energy (hν–hν0) is transferred as the kinetic energy of the photoelectron. Following the conservation of energy principle, the kinetic energy of the ejected electron is given by
Energy of photon = Threshold energy+ Kinetic energy
hv=hv0+KE
Where, v0= threshold frequency
v= frequency of the photon striking the surface
Photoelectric effect states that when specific metals are exposed to a sufficiently energetic light beam, the metal ejects the electrons.
This phenomenon is known as the photoelectric effect. The electrons ejected are called photoelectrons.
Explaining the result of the photoelectric effect.
i) The electrons are ejected from the metal surface as soon as the beam of light strikes the surface.
ii) The number of electrons ejected is proportional to the intensity of radiation.
iii) For each metal; there is a minimum frequency, below which photoelectric effect is not observed. This frequency is known as threshold frequency.
iv) Kinetic energy of electrons is proportional to the frequency of light.
Explanation of the photoelectric effect on the basis of quantum theory
Einstein was able to explain the photoelectric effect using Planck’s quantum theory of electromagnetic radiation as a starting point
Shining a beam of light on to a metal surface can, therefore, be viewed as shooting a beam of particles, the photons. When a photon of sufficient energy strikes an electron in the atom of the metal, it transfers its energy instantaneously to the electron during the collision and the electron is ejected without any time lag or delay.
Greater the energy possessed by the photon, greater will be transfer of energy to the electron and greater the kinetic energy of the ejected electron. So, considering photon as particles on basis of quantum theory does explain the results of photoelectric effect.
Since the striking photon has energy equal to hν and the minimum energy required to eject the electron is hν0(also called work function, W0),
then the difference in energy (hν–hν0) is transferred as the kinetic energy of the photoelectron. Following the conservation of energy principle, the kinetic energy of the ejected electron is given by
Energy of photon = Threshold energy+ Kinetic energy
hv=hv0+KE
Where, v0= threshold frequency
v= frequency of the photon striking the surface
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