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Discussion

As the current through the filament is increased, the temperature of the filament increases from room temperature to about 800 K, when the filament begins to glow dull red, and finally to a maximum of about 2800 K, when it is most bright and nearly "white hot." At 800 K, the light intensity is low and long wavelength red light predominates. As the filament temperature increases, the overall light intensity increases and light of shorter wavelengths (green through blue) is most intense.

In 1900 the German physicist Max Planck was able to explain this temperature dependence of the spectrum of an incandescent source by assuming that energy could be absorbed and emitted only in discrete amounts, called quanta. Five years later Albert Einstein extended Planck's theory by proposing that electromagnetic radiation, including visible light, consisted of discrete packets of energy, now called photons.