PHOTOELECTRIC EFFECT
Each photon delivers its entire energy h f to a single electron in the material. For
an electron to be ejected from the material, the photon’s energy must be greater
than the energy binding the electron to the material. If the photons energies are
less than the binding energies, zero electrons can be emitted from the material,
irrespective of how intense the incident light beam. Hence, using the principle of
conservation of energy
Energy before (photon) E
= Energy after (ejection of electron from material W + K.E. of ejected
electron EK)
E  h f  W  EK
When the energy required to remove an electron from the material is a minimum
Wmin, (Wmin is the work function of the material), the kinetic energy of the ejected
electron will be a maximum EKmax, hence,
h f  Wmin  EK max
h f  Wmin  12 me vmax 2
The applied potential can be used to retard the electrons from reaching the
collector. The retarding voltage, when the photocurrent I becomes zero, is called
the stopping voltage and its value can be used to measure the maximum kinetic
energy of the photoelectrons
eVS  12 me vmax 2
Measurements of EKmax and f can be used to measure Planck’s constant h by
plotting EKmax vs f. This gives a straight line with intercept Wmin and slope h since
EKmax = hf - Wmin
Electrons emitted only when incident frequency f greater or equal to critical frequency fC
photoelectrons when f  fC
fC = Wmin / h
If f  fC doubling incident light intensity  doubles the photocurrent but does not
increase the kinetic energy of the emitted electrons
Equation Mindmap eq21
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Equation Mindmap eq21
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max KE of ejected
electron EKmax
incident photon E = h f
bound electron
all photon energy
acquired by electron:
min energy required
to remove electron
from material Wmin
(work function)
Conservation of energy:
h f  Wmin  12 me vmax 2
Electron ejected
from surface
without delay
(“kicked-out”)
photoelectron
ejected from
material
incident photon Eblue = h fblue
energy
absorbed
by electron
bound electron
fblue > fred
hfblue > Wmin hfred < Wmin
incident photon Ered = h fred
bound electron
energy
absorbed by
material
insufficient energy
acquired to eject
electron
Equation Mindmap eq21
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