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Blackbody Radiation. A blackbody is something that absorbs all radiation that shines on it Are all blackbodies black? no!! imagine a box full of lava. A constant temperature blackbody (a.k.a. a very precise oven). Order your own blackbody online. Blackbodies and Astronomy.

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blackbody radiation
Blackbody Radiation
  • A blackbody is something that absorbs all radiation that shines on it
  • Are all blackbodies black?
    • no!!
    • imagine a box full of lava

A constant temperature blackbody

(a.k.a. a very precise oven)

Order your own blackbody online

blackbodies and astronomy
Blackbodies and Astronomy
  • Stars are very similar to blackbodies
    • emit a continuous spectrum of radiation
  • Why aren’t they black?
Blackbodies emit light at all wavelengths
  • Cooler blackbodies emit more red than blue light.
planck function
Planck Function

Iν(T) is the specific intensity, depends on T and ν

T is temp in Kelvin (Kelvin = Celsius + 273.2)

his Planck’s constant: 6.636 x 10-34 J s

k is Boltzmann’s constant 1.38 × 10-23 m2 kg s-2 K-1

c is the speed of light

rayleigh jeans limit
Rayleigh Jeans Limit
  • At low frequencies, hν << kTso the Planck function can be approximated as
properties of the planck law
Properties of the Planck Law
  • Wien Displacement Law:lmT = b, where lmis the wavelength at which Ilpeaks, and b (=0.0029 m K) is called the Wien displacement law constant.

Alternatively, νm/ T = 5.88 x 1010Hz K-1, where νmis the

frequency at which Iνpeaks.

Note that

wien s law
Wien’s Law
  • The peak wavelength of a blackbody spectrum is inversely proportional to temperature:

Temperatures of stars and planets are measured using Wien’s law.

which star is the hottest
Which star is the hottest?

Star B


Star D

Star C



properties of the planck law1
Properties of the Planck Law
  • Stefan-Boltzmann law:F = sT4, where F is the total radiated power per unit area (W per square m) and sis called the Stefan-Boltzmann constant: 5.67 x 10-8 W m-2 K-4.
stefan boltzmann law
Stefan-Boltzmann Law:
  • Hotter blackbodies emit more total energy
    • notice the area under the blackbody curve:

5000 K

4000 K

A perfect blackbody produces a continuous spectrum:
  • Dark lines in solar spectrum are from absorption by Sun’s outer atmosphere
emission line spectra
Emission Line Spectra
  • Take a thin cloud of gas composed of a pure element (e.g. hydrogen) and heat it to high temperature
    • It does not emit a continuous spectrum.
  • It emits light at specific wavelengths:
    • the exact same ones at which it absorbs
the role of density
The role of density
  • If thin gases produce only emission lines, and the Sun is made of gas, why does the Sun’s spectrum look continuous (like a rainbow?)

Solar spectrum



A high-density gas cloud produces a continuous spectrum