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Light, an electromagnetic radiation, is a combination of various wavelengths -- individual colours of light -- in various amounts. The visible wavelengths range from 400 to 700 nm, with >700 nm being infrared and <400 nm being ultraviolet. Blackbodies are objects that can emit and absorb electromagnetic radiation of all frequencies. Most objects are blackbodies that emit infrared radiations. We just do not see it since infrared radiation is invisible to the human eye.

Wilhelm Wien, a German physicist, was the first person who wrapped his head around the relationship between the temperature of a blackbody and the wavelength at which it emits the most light. He sent electromagnetic radiations through a hole he made on an oven and captured them so that all the wavelengths from an incoming radiation were measurable. He then changed the temperature of the oven to observe any changes in radiation. He found that, as the temperature (in Kelvin) increased, the wavelength (in metres) at which the radiations were strongest decreased, and that the product of the two values is 0.002898 metres Kelvin.

The temperature of light could not be measured directly, but the voltage could be changed using the variable voltage power supply. The intensities of different wavelengths produced from the diffraction grating were measured with the as the voltage changed.

The results were consistent with Wien's Law, especially in lower voltages (temperatures).