The spectrum of a blackbody, which represents the amount of light emitted at each wavelength, depends entirely on its temperature.

Everything emits Electromagnetic (EM) radiation. The sun does, operating incandescent lights do. We do. (Stand near an athlete who has been working out for an hour. You can feel the EM radiation (heat) being radiated by the athlete) Buildings do. (Stand near a brick building’s west wall just after sunset. You can feel the EM radiation being radiated). Anything at a higher temperature than its surroundings radiated EM radiation. The amount of power liberated in the form of EM radiation, net radiation, depends on the object’s temperature in Kelvin, and its size in m2. As you will find out, the wavelength of the EM radiation is related to temperature.

Note:    Any body at any temperature emits radiation even when it’s colder than its surroundings!

                Heat exchange from colder body to warmer body and vice versa until equilibrium.

 

 

Measuring the intensity of radiation corresponding at different wavelength

Heating the body to different temperature they made a number of curves.

The results of the experiment are plotted graphically and are shown in Figure.4.

Each curve represents the variation of monochromatic emittance with wavelength λ.

Can be observed that the higher the temperature of a black body is, the more energy is emitted in each band of wavelengths. The black body becomes "brighter". Moreover, the radiation emitted at the highest intensity, represented by the peak of the spectrum, doesn't go in the visible region unless the temperature is very high.