Sonoma State University
J. S. Tenn
Planck’s Derivation of the Energy Density of Blackbody Radiation To calculate the number of modes of oscillation of electromagnetic radiation possible in a cavity, consider a onedimensional box of side L. In equilibrium only standing waves are possible, and these will have nodes at the ends x = 0, L. L = n x n x = 1, 2, 2 and since = c = speed of propagation for all wave motion, = nx c 2L There will be two modes for each trio of integers (one for each dimension) nx, ny, nz because there are two independent polarizations possible. To find the number of modes with frequency between and + d , look at an array of points: z
y
Each cube has side c . 2L
x
There is one point per cube of volume c 3, and only positive integers, nx, ny, nz are acceptable. 2L Thus the number of triplets of positive integers is equivalent to the volume of one octant of the space divided by the volume c 3: 2L 2 × 1 × 4π 2 d no. modes of oscillation = 8 = 8π V 2 d between and + d c 3 c3 2L The factor 4π 2 d is the volume of a thin spherical shell; L3 has been replaced by V, the volume of the cavity. It is convenient to express this density of states in terms of other variables: g( ) d = 8 V 2 d g( ) d = 8 V d 4 c3 g( ) d = 8 V 2 d g(p) dp = 8 V p 2 dp 3 hc h3 = h = pc The expressions involving frequency energy , and wavelength are classical physics. If we assume that each mode of oscillation represents a harmonic oscillator, with 1kT each potential 2 and kinetic energy on the average (in accordance with the equipartition theorem), we get the RayleighJeans law: Energy Energy ≡ u d = 8 kT 2 d or ≡ u d = 8 kT d 4 Volume Volume c3 The divergence of this relation at high frequency or low wavelength was known as the ultraviolet catastrophe. Planck’s new idea was to assume that the possible energies of the oscillators were quantized, i.e., that oscillators of frequency could only have energy n = 0, 1, 2, n = nh where h was a new constant he introduced. Now known as Planck’s constant, it was determined by fitting the theoretical curve to the experimental data. The average energy per oscillator was calculated from the MaxwellBoltzmann distribution: ∑ n e  n/kT = n ∑ e  n/kT n
Note on black body radiation
p. 2
The denominator is called the partition function, and is often represented by Z. It is easily evaluated by summing the geometric series: ∞ ∞ Z = ∑ e  n/kT = ∑ e nx = 1 x where x = h 1 e kT n=0 n=0 The numerator can then be found from the denominator: ∞ x ∑ n h e nx = h  dZ = h e 2 dx x n=0 1 e and the average energy per oscillator is seen to be h h = ex  1 e h /kT  1 Thus the energy per unit volume of the radiation in the cavity is h 3 d 1 u (T) d = 8 or u (T) d = 8 hc d 5 e hc/ kT  1 c 3 e h /kT  1 The total energy per unit volume (energy density) is the integral over all frequencies or wavelengths: =
u (T) = 8 h c3
∞
3
eh
/kT
1
d
=
8 kT 3
4
∞
x 3 dx ex  1
(hc ) 0 4 The integral is obviously a pure number. It happens to be /15. Thus the energy density in a black body is 8 5 kT 4 u (T) = ≡ aT4 3 15(hc ) This may be thought of as one form of the StefanBoltzmann law. [Josef Stefan in 1879 showed experimentally that the flux from a cavity in thermal equilibrium is proportional to the fourth power of the absolute temperature, and Ludwig Boltzmann in 1884 derived this fourth power relation from thermodynamic theory. Until Planck’s work, there was no theoretical method of determining the constants of proportionality.] 0
The flux radiated from the surface of a black body is related to the energy density: 2 1 h 3 = 2 or F = c u = 2 hc 2 h /kT 5 hc/ kT  1 4 c e 1 e energy where F d = flux = with frequency between and + d . area ⋅ time The total flux, obtained by integrating over all frequencies, is
F = cu 4
F = c u = ac T 4 ≡ T 4 4 4 This is the usual form of the StefanBoltzmann law. The constant erg = 5.670 × 108 W = 5.670× 105 = StefanBoltzmann constant. 4 m2 K cm 2 s K4 It is of interest to look at the limits of the Planck distribution. At low frequency or large wavelength, u (T) → 8
2kT
c3
and
u (T) → 8 kT = RayleighJeans law. 4
Note that Planck’s constant drops out. This is one example of the correspondence principle; as h becomes negligible compared to other quantities in the quantum mechanical law, the result approaches the classical law. At high frequency or small wavelength, 3 u (T) → 8 h e h /kT and u (T) → 8 hc e hc/ kT 3 5 c The frequency or wavelength of maximum flux can be found by setting the derivative with respect
Note on black body radiation
to
or
p. 3
equal to zero: 1 0 = dF = 2 h 3 2 h /kT d c e 1
2

3(h/kT)e h /kT
eh
/kT
1
x which simplifies to 3 = xe with x = h ex  1 kT The transcendental equation may be solved graphically [graph y = x and y = 3(1  ex) and find the nonzero point of intersection] or numerically. The result is x = 2.821 meaning that flux as a function of frequency is a maximum at max = 2.821 kT/h. Maximization of F is similar: ye y 0 = dF ⇒ which yields y = 4.965. = 5 with y = hc d ey  1 kT Flux as a function of wavelength is a maximum at
2.90 mm ⋅K hc = . 4.965kT T At room temperature T ≅ 290 K, and thermal radiation is a maximum at ≅ 0.01 mm = 10 µm, in the infrared. Fortunately, our eyes are not sensitive to this wavelength. The maximum intensity of the sun’s radiation is at ≅ 500 nm, implying that the sun’s surface temperature is T ≅ 5800 K. The variation of intensity with wavelength of the sun [and other stars] is not exactly that of a black body, but it is rather close. The universal microwave background radiation, peaked at ≅ 1 mm, fits the Planck curve for a black body of T = 2.728 K to great precision. (The deviation, of order 6 parts in 106 is, of course, of great interest.) max
=
Illustration source: http://www.bc.cc.ca.us/programs/sea/astronomy/light/lightb.htm#A2.1
Now at http://www.astronomynotes.com/light/s4.htm