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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.

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Blackbody Radiation

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  1. 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

  2. Blackbodies and Astronomy • Stars are very similar to blackbodies • emit a continuous spectrum of radiation • Why aren’t they black?

  3. Blackbodies emit light at all wavelengths • Cooler blackbodies emit more red than blue light.

  4. 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

  5. Rayleigh Jeans Limit • At low frequencies, hν << kTso the Planck function can be approximated as

  6. 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

  7. 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.

  8. Which star is the hottest? Star B Wavelength Star D Star C Blue Red

  9. 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.

  10. Stefan-Boltzmann Law: • Hotter blackbodies emit more total energy • notice the area under the blackbody curve: 5000 K 4000 K

  11. A perfect blackbody produces a continuous spectrum: • Dark lines in solar spectrum are from absorption by Sun’s outer atmosphere

  12. 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

  13. 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

  14. Low-density A high-density gas cloud produces a continuous spectrum

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