As a blacksmith heats a piece of iron, the iron glows red, then yellow, then white. The iron provides a demonstration of which phenomenon?

A. Emission spectra

B. Quantum mechanics

C. Photoelectric effect

D. Blackbody radiation

The answer is D. Blackbody radiation. The piece of iron glows red because its temperature is around 1000 K, then yellow because its temperature is around 2800 K, and then white because its temperature is around 5500K. This shows that the spectrum of the radiation is determined by absolute temperature, as when the temperature of a blackbody radiator increases, the peak of the radiation curve moves to shorter wavelengths.