1090. How does the temperature of a fire correspond to its color. How hot is blue fire? How hot is yellow fire? — SF, Lake Almanor, CA
The hotter the fire, the more green and blue light it emits. The dimmest glow that you can see in a darkened room appears when a surface is about 400° C. The dull red of a heat lamp is about 500° C. A candle's yellow glow is about 1700° C. A normal incandescent lamp is about 2500° C. And the sun is about 5800° C. Blue fire would be hotter still, except it's usually colored artificially by the presence of excited atoms. Atomic emissions are colored because atoms can't emit all colors in order to produce a normal spectrum of thermal radiation. Instead, they preferentially emit only specific colors. That's why when you burn copper, you see blue-green light, even when the copper isn't very hot. The copper atoms just can't emit red or yellow light, even though those would be the more appropriate colors at the temperature of the burning copper.