lec13 - Lecture 13 Blackbody Radiation Lecture 13 Purdue...

Lecture 13
Blackbody Radiation
Lecture 13 Purdue University, Astronomy 364 1

Specific Intensity
dA
Normal
d
The amount of power passing through an element along its
normal direction per unit area per unit solid angle per unit
frequency range
The amount of energy crossing dA with a solid
angle d  in time dt and in frequency range d  is
given by
dE = I
ν
⋅ dA ⋅ dt ⋅ d Ω⋅ d ν
Specific intensity or brightness
I

is in units of
J s
− 1
m
− 2
ster
− 1
Hz
− 1

Mean Specific Intensity
• Total Intensity (or simply, intensity)
• Mean Specific Intensity
Lecture 13 Purdue University, Astronomy 364 3
I = I
ν
d ν
0
∞
∫
J
ν
=
1
4 π
I
ν
d Ω
∫
For isotropic radiation, J

= I


Blackbody Radiation
T
Imagine a cavity whose wall is
heated up to a temperature T.
The radiation emitted by the
wall is trapped inside the
cavity. If the temperature is
uniform over the wall, the
system is said to be in
thermodynamic equilibrium.