Blackbody Radiation

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Blackbody Radiation
Blackbody ideal thermal object that absorbs all
radiation falling upon it at low temperatures.
Plotting radiation vs ???????? a curve!
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Area under curve increases with Temperature!
I
4000 K
Classical theory
3000 K
2000 K
?
Area radiation absorbed
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Why is it that as the ???????????? zero, the
amount of energy carried by short wavelength
radiation approaches zero?????
UltraViolet Catastrophe
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Area under curve increases with Temperature!
I
4000 K
Classical theory
Visable light range
3000 K
2000 K
?
Area radiation absorbed
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Photoelectric effect
Light shining on metal may eject electrons from
the metal.Because this interaction involves both
light and electrons, it is called the
phototelectric effect!
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Photoelectric Effect
1.For a given metal, light must have a minimum
frequency , called the cutoff frequency or
electrons will not emitt. 2.The KE of the
photoelectrons is not dependent upon light
intensity. 3.The max KE of the photoelectrons
increases with increasing light
frequency 4.Electrons are emitted almost
immediately from the surface
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The Photoelectric Effect
Classical
Experimental
Electrons are Ejected based on
Intensity of the light
Frequency of the light
Ejected electrons KE Depends on
Frequency of the light
Intensity of the light
At low intensities, Electron ejection
Takes Time
Happens Instantly
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Planck Rules!
1.The vibrating molecules emitting radiation can
do so only in discrete bundles of energy .
En nhf h 6.626x 10-34 J s 4.14 x10
-15 eV s
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Planck Rules!
2.Molecules emit energy in units called quanta
(photons). They do this by jumping from one
energy state to the other. The energy of light
emmitted by jumping states is given by..
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because
then
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Work Function and Electron ejection
Fact no electrons are emitted if the incident
light frequency falls below a cutoff frequency!
Why? THE WORK FUNCTION ? The WF
represents the minimum energy with which an
electron is bound in the metal!!
then
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Work Function and Electron ejection
then
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Core Problem 1
A sodium surface is illuminated with a frequency
of 1. 00 x 10 15 Hz. The workfunction of sodium
is 2.28 eV. Find the maximum KE of the
photoelectrons in eVs!!! (Hint 1.60 x10 -19
J/eV)
then
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Core Problem 1
A sodium surface is illuminated with a frequency
of 1. 00 x 10 15 Hz. The workfunction of sodium
is 2.28 eV. Find the maximum KE of the
photoelectrons in eVs!!!
then
6.63x 10 -34 Js (1.00 x 10 15Hz) - (2.28eV) 1.60
x 10 -19J/eV KE1.86 eV
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Work functions of selected metals
Metals ?????
Na 2.46 Al 4.08 Cu 4.70 Zn 4.31 Ag 4.73
Pt 6.35 Pb 4.14
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Compton Effect
Proof of particle theory Scattered photons
involving collisions with X-ray photons show less
energy than the original photon! Why? momentum
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De Brogile Waves
2
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