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In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature . As a function of frequency , Planck's law is written as: This function represents the emitted power per unit area of emitting surface, per unit solid angle, and per unit frequency. Sometimes, Planck's law is written as an expression for emitted power integrated over all solid angles. In other cases, it is written as for energy per unit volume. The function peaks for h = 2.82kT. It falls off exponentially at higher frequencies and polynomially at lower.

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  • Planck's law
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  • In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature . As a function of frequency , Planck's law is written as: This function represents the emitted power per unit area of emitting surface, per unit solid angle, and per unit frequency. Sometimes, Planck's law is written as an expression for emitted power integrated over all solid angles. In other cases, it is written as for energy per unit volume. The function peaks for h = 2.82kT. It falls off exponentially at higher frequencies and polynomially at lower.
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  • In physics, Planck's law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black body at temperature . As a function of frequency , Planck's law is written as: This function represents the emitted power per unit area of emitting surface, per unit solid angle, and per unit frequency. Sometimes, Planck's law is written as an expression for emitted power integrated over all solid angles. In other cases, it is written as for energy per unit volume. The function peaks for h = 2.82kT. It falls off exponentially at higher frequencies and polynomially at lower. As a function of wavelength λ, Planck's law is written (for unit solid angle) as: This function peaks for hc = 4.97λkT, a factor of 1.76 shorter in wavelength (higher in frequency) than the frequency peak. It is the more commonly used peak in Wien's displacement law. The radiance emitted over a frequency range or a wavelength range can be obtained by integrating the respective functions. The order of the integration limits is reversed because increasing frequencies correspond to decreasing wavelengths. The following table provides the definition and SI units of measure for each symbol: {| class="wikitable" |- !Symbol !Meaning !SI units !cgs units |- | | spectral radiance, or energy per unit time per unit surface area per unit solid angle per unit frequency or wavelength (as specified) |J·s-1·m-2·sr-1·Hz-1, or J·s-1·m-2·sr-1·m-1 |erg·s-1·cm-2·Hz-1·sr-1, or erg·s-1·cm-2·sr-1·cm-1 |- | |frequency |hertz (Hz) |hertz |- | |wavelength |meter (m) |centimeter (cm) |- | |temperature of the black body | kelvin (K) |kelvin |- | |Planck constant |joules · second (J·s) |ergs · second (erg·s) |- | |speed of light |meters per second (m/s) |centimeters per second (cm/s) |- | |base of the natural logarithm, 2.718281... |dimensionless |dimensionless |- | |Boltzmann constant |joules per kelvin (J/K) |ergs per kelvin (erg/K) |}
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