Shouldn't the vacuum insulate the glass from the heat of the burning filament?

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[–] 2 points 3 years ago

Yes, as you heat something up to "red hot", the glow shifts from infrared to being partly in visible red frequencies. This is why a blacksmith can use the color of a piece of hot iron to tell how hot it is.

https://en.wikipedia.org/wiki/Black-body_radiation

(This isn't the only way hot things make light, though — for instance, flames can glow with odd colors like green or blue due to specific chemicals burning.)

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