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An increasing voltage is applied to an unfrosted lamp bulb placed behind a diffraction grating. As the current increases, the temperature of the lamp filament increases. At about 800 K the filament begins to glow dull red and the visible spectrum of the radiation appears. As the voltage increases further, a maximum of about 2800 K is reached, when the filament is brightest and nearly "white hot."

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At 800 K, the light intensity is low and long wavelength red light predominates. As the filament temperature increases, the overall light intensity increases with light of shorter wavelengths (green through blue) dominating the spectrum. The German physicist Max Planck was able to explain this temperature dependence of the spectrum by assuming that energy could be emitted only in discrete amounts, called quanta.