If incandescent lightbulbs have a vacuum inside, why do they get so hot on the outside?

Epic2112@lemmy.world to No Stupid Questions@lemmy.world – 112 points –

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

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Heat is infrared. Light. Vacuum doesn't have much effect.

Also many bulbs are filled with inert gases rather than being vacuums.

Heat is infrared. Light.

All light heats up anything that absorbs it. This includes infrared, but it also includes visible light, microwaves, radio waves, etc. You can get a nasty burn from putting your hand near a live radio transmitter antenna, for example, even though it's emitting in RF, not infrared.

In addition, all physical objects glow with a light that is determined by their temperature. This includes your body. You are, right now, emitting light. As it happens, because of your body's temperature, that light is mostly in the infrared.

Why do kids' science books leave you with the impression that "heat is infrared"? Because you can see body heat with an infrared camera. Infrared is light that you can't see with your eyes — but with the right tool, you can use to see body heat. This rounds off to "heat is infrared".

Heat is not infrared. All physical objects emit light; objects around human body temperature glow mostly in the infrared; which we can't see with our eyes, but can see with scientific instruments. And when an object absorbs light (including infrared), it gets hotter.

If I super heat a metal and it turns visibly red what is happening? Was it already emitting infrared and as it gets hotter the frequency shifts up? Or is it still emitting infrared but has a wider band of frequencies it is emitting as well (i.e. is it emitting frequencies below infrared as well as visible red)?

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.)