Blackbody Kirchhoffs Radiation Radiated electromagnetic energy is the
Blackbody
Kirchhoff’s Radiation ] Radiated electromagnetic energy is the source of radiated thermal energy. • Depends on wavelength ] Objects can emit and absorb electromagnetic energy. • Emission coefficient el • Absorption coefficient al ] Expect a distribution Il that depends on temperature.
Black Body ] 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).
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.
Radiation Spectrum ] The frequency spectrum power is a function of temperature. • Wl(l, T) ] Earth surface: 300 K 20 ºC ] Sun surface: 5800 K 6100 ºC intensity low energy high energy ] Sun interior: 1. 57 107 K frequency
Ultraviolet Catastrophe ] Classical thermodynamics predicted that the power emitted would increase at higher frequency. • Infinite energy emitted ] Real data did not match this intensity low energy high energy conclusion. frequency
Quanta of Light ] Accelerated charges emit electromagnetic waves. ] Planck assumed that oscillating charges emit only discrete energies (1900). • Energy is quantized. Emitted photon Moving charge
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
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