TED KINSMAN / SCIENCE PHOTO LIBRARY TED KINSMAN / SCIENCE PHOTO LIBRARY
A light bulb filament is heated by different voltages. The tungsten in the filament changes colour from red to white as the temperature rises, in a process called thermionic emission of light. The colour of the light given off by the top metal is called the colour temperature. It glows in proportion to the electrical current is passed through the tungsten filament. Image 3 in a series of 4.
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