Blackbody. Kirchhoff’s Radiation  Radiated electromagnetic energy is the source of radiated thermal energy. Depends on wavelengthDepends on wavelength.

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Presentation transcript:

Blackbody

Kirchhoff’s Radiation  Radiated electromagnetic energy is the source of radiated thermal energy. Depends on wavelengthDepends on wavelength  Objects can emit and absorb electromagnetic energy. Emission coefficient Emission coefficient  Absorption coefficient Absorption coefficient   Expect a distribution I that depends on temperature.

 A black object is perfectly absorbing. Absorption coefficient is 1  The distribution is just due to emission.  An isolated cavity with a narrow hole radiates like a perfectly black body at the same temperature (1859). Black Body

Blackbody Power  The total power from a blackbody is defined by the power per unit area. W/m 3  The power radiated varied with temperature. Fourth power (1879) Stefan-Boltzmann law (1884).  Real objects have a factor for emissivity.

low energy high energy frequency intensity Radiation Spectrum  The frequency spectrum power is a function of temperature. W (,T)W (,T)  Earth surface: 300 K  20 ºC  Sun surface: 5800 K  6100 ºC  Sun interior: 1.57  10 7 K

Ultraviolet Catastrophe  Classical thermodynamics predicted that the power emitted would increase at higher frequency. Infinite energy emittedInfinite energy emitted  Real data did not match this conclusion. low energy high energy frequency intensity

Quanta of Light  Accelerated charges emit electromagnetic waves.  Planck assumed that oscillating charges emit only discrete energies (1900). Energy is quantized.Energy is quantized. Moving charge Emitted photon

Planck’s Radiation  Planck established a relationship between the energy and frequency. Planck’s constant h  Planck solved the Kirchhoff problem and UV catastrophe. next