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Photoelectric effect - (Concept)
Whenever a metal surface is exposed to light radiation of suitable energy or frequency, it was observed that some electrons get ejected from the metal surface. This phenomenon is called as Photoelectric effect and the ejected electrons are called as photoelectrons.
There were certain observation in the photoelectric effect experiment.
(1) There was a requirement of a minimum energy for each metal for the photoelectric effect to occur. This minimum energy is known as work function (W0) and it can be closely associated with the ionisation energy of the metal.
- Corresponding to the work function, there is a minimum frequency of light required for photoelectric effect. This minimum frequency is called as Threshold frequency.
- Corresponding to the work function, there is a maximum wavelength of incident light above which Photolelectric effect cannot occur. This maximum wavelength is called as the Threshold wavelength.
Mathematically, the work function, threshold frequency and threshold wavelength can be associated as
Note: hc is approximately equal to 2 10-25 J-m or 12400 eV-nm. (eV is the energy in electron volts)
(2) The number of electrons ejected is proportional to the intensity (brightness) of light striking the metal but does not depend upon the frequency of light.
(3) There was almost no time lag between the striking of light and ejection of photo electrons
(4) The kinetic energy of the ejected electrons (photoelectrons) depend upon the frequency of the light used.
Einstein's photoelectric equation
From conservation of energy
where
m is the mass of the electron
v is the velocity associated with the ejected electron.
h is planck’s constant.
v is frequency of photon,
v0 is threshold frequency of metal.
(5) The Kinetic energy of ejected photoelectron is also sometimes associated to Stopping Potential. It is defined as the minimum opposing potential applied due to which kinetic energy of electron becomes zero.
where,
Vs = Stopping potential
e = Charge on electron
| Exam | Chapter |
| JEE MAIN | Atomic Structure |
Light of wavelength shines on a metal and emits Y electrons per second of average energy Z. What will happen to Y and Z if intensity of light is doubled?
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Which of the graphs shown below does not represent the relationship between incident light and the electron ejected from metal surface ?
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If and
be the threshold wavelength and wavelength of the incident light, the velocity of the photoelectron ejected from the metal surface is :
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Ejection of the photoelectron from metal in the photoelectric effect experiment can be stopped by applying 0.5 V when the radiation of 250 nm is used. The work function (eV) of the metal is :
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Electromagnetic radiation of wavelength 663 nm is just sufficient to ionise the atom of metal A. The ionization energy of metal A in kJ mol–1 is _______. (Rounded-off to the nearest integer)
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A metal surface is exposed to solar radiations
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The metal mainly used in devising photoelectric cells is :
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The work function of sodium metal is , if photons of wavelength 300 nm are incident on the metal , the Kinetic Energy of the ejected electrons will be
___________
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The minimum energy that must be possessed by photons to produce the photoelectric effect with platinum metal is:
[Given: The threshold frequency of platinum is and
]
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If the work function of a metal is , the maximum wavelength of the photon required to remove a photoelectron from the metal is ________ nm. (Nearest integer)
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A source of monochromatic radiation of wavelength provides
of energy in 10 seconds. When this radiation falls on the surface of sodium,
electrons are ejected per second. Assume that wavelength
is sufficient for ejection of electron from the surface of sodium metal. The value of x is ________(Nearest integer)
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A metal surface is exposed to 500 nm radiation.The threshold frequency of the metal for photoelectric current is . The velocity of ejected electron is _______
.(Nearest integer).
[Use : ]
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Light of wavelength shines on a metal emits y electrons per second of average energy z. What will happen to y and z if Intensity of light is doubled?
| A. |
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| B. |
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| C. |
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| D. |
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If P is the momentum of the fastest electron ejected from a metal surface after the irradiation of light having wavelength , then for 1.5 p momentum of the photoelectron, the wavelength of the light should be:
(Assume kinetic energy of elected photoelectron to be very high in comparison to work function):
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The energy of a photon in Joules that has a wavelength of 9.0m is:
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If P is the momentum of the fastest electron ejected from a metal surface after the irradiation of light having a wavelength 
(Assume kinetic energy of elected photoelectron to be very high in comparison to work function):
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| B. |
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| C. |
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| D. |
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he work function of sodium metal is , if photons of wavelength 300nm are incident on the metal , the Kinetic Energy of the ejected electrons will be
___________
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The figure that is not a direct manifestation of the quantumnature of atom is :
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For a metal surface, the work function is 3.5 eV. If the incident light has a wavelength of 400 nm, what is the kinetic energy of the emitted electrons?
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Assertion: If the frequency of incident light is below the threshold frequency, no electrons will be emitted from the metal surface, irrespective of the intensity of the incident light.
Reasoning: The photoelectric effect equation states that the kinetic energy of the emitted electrons depends on the frequency of the incident light and the work function of the metal surface. If the frequency of incident light is below the threshold frequency, the energy of the photons will be insufficient to overcome the work function of the metal surface and no electrons will be emitted.
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The energy of one mole of photons of radiation of frequency is . (Nearest integer)
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Given below are two statements : one is labelled as Assertion and the other is labelled as Reason
:
Assertion : In the photoelectric effect electrons are ejected from the metal surface as soon as the beam of light
of frequency greater than threshold frequency strikes the surface.
Reason : When the photon of any energy strikes an electron in the atom transfer of energy from the photon to
the electron takes place.
In the light of the above statements, choose the most appropriate answer from the options given below:
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Values of work function for a few metals are gives below
The number of metals which will show photoelectric effect when light of wavelength 400 nm falls on it is___________
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What is the shortest distance between nucleus of and the point where an
particle with kinetic energy Mev falls on the foil made up of aluminium?
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The ionization energy of sodium in $\mathrm{kJ} \mathrm{mol}^{-1}$, if electromagnetic radiation of wavelength $242 \mathrm{~nm}$ is just sufficient to ionize sodium atom is________
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In 1887, H. Hertz performed a very interesting experiment in which electrons (or electric current) were ejected when certain metals (for example potassium, rubidium, caesium etc.) were exposed to a beam of light as shown in Fig.2.9. The phenomenon is called Photoelectric effect.
