THE PHOTOELECTRIC EFFECT

1
THE PHOTOELECTRIC EFFECT
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When red light is incident on a clean metal
surface
Clean metal surface
e
e
e
e
e
e
no electrons are released, however long light is
shone onto it, however intense the light source
is.
3
When UV light is incident on a clean metal
surface
UV light
Clean metal surface
e
e
e
e
e
e
e

• electrons are released instantaneously,
• however weak the light source.

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Classically this cannot be explained because

• If red light is shone onto the metal surface
• for long enough some electrons should
• gain sufficient energy to enable them to escape.

5
Einstein put forward a theory

• Light energy is quantized.
• Light consists of a stream of particles called
  photons.
• The energy of each photon (E) depends
  
• on the frequency (f ) of the light.

h x
f
E
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h
is Planck's constant
red light has a smaller
frequency
Frequency increasing
than violet light
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E h x f
Photon energy
Red light photons therefore
have less energy
than violet light photons
and even less than UV photons
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ONE PHOTON
GIVES ALL ITS ENERGY
e
TO ONE ELECTRON
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A photon of red light gives an electron
insufficient energy to
enable it to escape from the surface of the metal.
Red light photon
Clean metal surface
e
e
e
e
e
e
e
surface electrons
No electrons are released from the metal surface.
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A photon of UV light gives an electron sufficient
energy to
enable it to escape from the surface of the metal.
UV photon
Clean metal surface
e
e
e
e
e
e
e
surface electrons
Electrons are released instantaneously. Each
photon releases an electron This is called
photoemission.
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BLACKBODY RADIATION
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Significance of Black Body

• The black body concept has a significant role in
  thermal radiation theory and practice.
• The ideal black body notion is important
  in studying thermal radiation and
  electromagnetic radiation transfer in all
  wavelength bands.
• The black body model is used as a standard with
  which the absorption of real bodies is compared.

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Definition of a black body
A black body is an ideal body which allows the
entire incident radiation to pass into itself
(without reflecting the energy ) and absorbs
within itself (without transmitting the energy).
This propety is valid for radiation corresponding
to all wavelengths and to all angles of
incidence. This renders a black body an ideal
absorber of incident radiation.
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The UV Catastrophe
Theory experiment disagree wildly
Pre-1900 theory
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Plancks solution
EM energy cannot be radiated or absorbed in any
arbitrary amounts, but only in discrete quantum
amounts. The energy of a quantum depends on
frequency as Equantum h f
h 6.6 x 10-34 Js
Plancks constant
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Quanta and the UV catastrophe
Without the quantum
With the quantum
Low frequency, small quantum, Negligible effects
high frequency, large quantum, huge effects
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Comparison between Classical and Quantum
Viewpoints
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Conclusion

• As the temperature increases, the peak wavelength
  emitted by the black body decreases.
• As temperature increases, the total energy
  emitted increases, because the total area under
  the curve increases.
• The curve gets infinitely close to the x-axis but
  never touches it.

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Black-body Radiation
2.9 x 10-3 m T(Kelvin)
l peak
Light intensity
UV
IR
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lpeak vs Temperature
2.9 x 10-3 m T(Kelvin)
T
l peak
310 K (body temp)
2.9 x 10-3 m 310 K
9x10-6m
infrared light
5800 K (Suns surface)
visible light
2.9 x 10-3 m 5800 K
0.5x10-6m