T H E P L A N C K F U N C T I O N
| Input a temperature value (in Kelvin), press return and the applet will show the Planck Spectrum (red) and the value of the peak wavelength (black line). Or, input a wavelength value and the applet will show the temperature and Planck spectrum of a blackbody that peaks at the given wavelength. |
Any object in the universe has a temperature and thus emits EM radiation at all wavelengths due to thermal motion (a probability distribution of kinetic energy) of charged particles. The intensity of the radiation as a function of wavelength is given by Planck's function.
To be precise, Planck's formula is only completely true for an idealized emitter (and absorber) known as a "blackbody." A blackbody would be completely absorbing at all wavelengths, reflecting no light.
In addition, a variety of non-thermal processes can produce EM radiation. And absorbtion and emission of light can add "line" features (peaks and valleys) to the spectrum. But the blackbody ideal is a very good baseline for most astronomical objects, such as stars.
The Planck spectrum has a peak at some wavelength which is given by Wien's formula. For instance, the photosphere of the Sun has a temperature of around 5800 Kelvin and thus peaks at 5000 Angstroms, a wavelength in the yellow region of optical (visible) light. Not by coincidence is the human eye most sensitive to the same wavelength. The human body at a normal temperature of about 310 Kelvin (98.6° F) peaks in the infrared. The Cosmic Background Radiation (CBR) at 2.7 Kelvin has a peak in the Microwave region.
An important property of the Planck function is that the blackbody spectrum of a hotter object lies entirely outside the spectrum of a colder one. Thus a hotter object radiates more at all wavelengths than a colder object.