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Question:
If the sun´s spectrum peaks in the green-yellow, why the measurement of the temperature of the projected spectrum gives the higher temperature in the infrared (Heschel´s experiment)?



Replies:
The infrared wavelengths of light are absorbed by most targets, so its energy is converted into vibrations of the target (heat). There are many variations on this principle. For example, a black target (e.g. sheet of paper) gets warmer faster than a white sheet of paper because the black sheet of paper absorbs all wavelengths of light and a white sheet of paper absorbs very little. It reflects most, while the black sheet reflects almost none. Once absorbed, the visible light is converted by a number of mechanisms into heat.

Vince Calder



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