Black Body Radiation
and
the experimental basis
for Quantum Theory
Josef Stefan (1835-1893)
• Total energy radiated proportional to T4
Question:
Suppose temperature raised
from 300K (room temp) to 6000K (sun)
By what factor does the energy radiated increase?
A.
B.
C.
D.
16
16,000
90,000
160,000
Josef Stefan (1835-1893)
• Total energy radiated proportional to T4
Question:
Suppose temperature raised
from 300K (room temp) to 6000K (sun)
By what factor does the energy radiated increase?
A.
B.
C.
D.
16
16,000
90,000
160,000 Answer: (6000/300)4 = 204=160,000
Steam Radiator 400K
Very Hot Stove 900K
Lava
~2000K
Sun
6000K
IR
Dull Red (transition to visable)
Glowing (most radiation still in IR)
Most radiation visable
Wilhelm Wien
Wilhelm Wien (18643-1928)
Nobel Prize 1911
Example of Black Body
Spectra for different
temperatures
Below is a photo of three stars. The light emitted by
these stars is thermal radiation. Which of these stars is
the hottest? The coolest?
A. Red, yellow
B. Red, blue
C. Yellow, blue
D.Blue, red
E. Blue, yellow
© 2007 W.W. Norton & Company, Inc.
Physics for Engineers and Scientists
9
Below is a photo of three stars. The light emitted by
these stars is thermal radiation. Which of these stars is
the hottest? The coolest?
A. Red, yellow
B. Red, blue
C. Yellow, blue
D.Blue, red
E. Blue, yellow
© 2007 W.W. Norton & Company, Inc.
Physics for Engineers and Scientists
10
Spectrum of high energy particle radiation from
space does not follow a black body spectrum
Cosmic Ray Flux
Above 1020 eV
Very low flux
~ 1 particle per
km2/sr/century
1 Joule
What is the best known example of a black body source?
What is the best known example of a black body source?
Hint
Temperature = 2.7 K
Cosmic Microwave Background (Radiation from Big Bang!
T=2.725K. The theoretical curve obscures the data points and the error bars.
Astrophysical Journal, 473, 576
Black Body Radiation
and
the experimental basis
for Quantum Theory
What is the energy of a “quanta” of RED light?
660 nm wavelength in units of electron volts?
E=hν
h=6.62x10-34 Js
What is the energy of a “quanta” of
RED light?
660 nm wavelength in units of
electron volts?
E=hν
h=6.62x10-34 Js
ν=c/λ=3x108m/s/660x10-9m
=4.54x1014 s-1
E= 6.62x10-34Js x 4.54x1014 s-1
E= 3.01 x 10-19 J
1eV= 1.602 x10-19 J
E=3.01x10-19J/1.602x10-19J/eV
E=1.88 eV
What is the energy of a “quanta”
of RED light?
660 nm wavelength in units of
electron Volts
Easier Way to Solve this
E=hc/λ
hc=1240 eV nm
(useful constant to remember)
E=(1240/660)eV
E= 1.88 eV
Question:
What is the energy quantization
of a grandfather clock?
Hint:
Question:
What is the energy quantization
of a grandfather clock?
Hint:
E=nhν
for n=1, ν=1Hz=1s-1 E= 6.6x10-34J
E=nhν
for n=1, ν=1Hz=1s-1 E= 6.6x10-34J
How is the quantization
realized?
E=nhν
for n=1, ν=1Hz=1s-1 E= 6.6x10-34J
How does this quantization
translate into quantization of
the pendulum displacement
(height)?
E=nhν
for n=1, ν=1Hz=1s-1 E= 6.6x10-34J
How does this quantization translate into
quantization of the pendulum displacement
(height)?
E=mgH=6.6x10-34J
H=6.6x10-34J/(1kg 10m/s2)=6.6x10-35m
H
Too small to measure (size of an atom is about 10-8 m)