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Radiation from which of the following sources approximates black body radiation best?

1. A tungsten lamp
2. Sodium flame
3. Hot lamp black
4. A hole in a cavity, maintained at constant temperature

Correct answer: A hole in a cavity, maintained at constant temperature

Solution

A hole in a cavity maintained at constant temperature is the best approximation of black body radiation because it absorbs all incident radiation and emits radiation characteristic of its temperature, closely following Planck's law.

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