Wien's Displacement Law

λ_max = 0.002898 / T

where T is the temperature of the black body in kelvin (K) and λ_max is the peak wavelength in meters. The 0.002898 is a proportionality constant with units m×K.

Basically, the hotter an object is, the shorter the wavelength at which it will emit radiation. For example, the surface temperature of the sun is 5780 K, giving a peak at 500 nm. This is fairly in the middle of the visual spectrum, due to the spread resulting in white light. Due to the Rayleigh scattering of blue light by the atmosphere this white light is separated somewhat, resulting in a blue sky and a yellow sun. A lightbulb has a glowing wire with a somewhat lower temperature, resulting in yellow light, and something that is "red hot" is again a little less hot.

Although the law was first formulated by Wilhelm Wien, today we it derive it from Planck's law of black body radiation.

Hint:

See also the "Learning by Simulations" Web site for an interactive simulation of black-body radiation.