Shouldn’t the vacuum insulate the glass from the heat of the burning filament?
The filament is heated by electrical resistance. That heat energy comes out as photons in a wide band in the visible and infrared parts of the spectrum. Some of those photons are intercepted by the glass bulb, the metal housing, etc.; their energy heats these materials up.
Even though a vacuum prevents conduction of heat energy, it doesn’t prevent radiation of that energy in the form of photons. That’s how the light gets from the filament to the room; and that’s how the heat gets to the surface of the bulb too.
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.
very closely related:
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.)
Heat radiates as infrared light. Infrared doesn’t pass through glass well, so the glass absorbs the heat radiated from the filament.
Heat passed through molecular collisions is conduction, and that is the part blocked by a vacuum.
So the filament is emitting visible and infrared and the bulb is designed to let the visible pass through.
There’s no perfect vacuum. But as others have mentioned already, most of it is electromagnetic radiation. A very small part of this radiation is the visible light you see, most of it is invisible to the human eye.
I’m no lightbulb expert but I can tell you a vacuum still allows heat transfer via radiation. This is how we get warm from the sun.
I also doubt the inside of a filament lightbulb is a near perfect vacuum, but maybe a bulb expert will come along to shed some light on that.
Heat radiation in a vacuum is also an important aspect of space travel. If heat could not radiate in a vacuum, we would not be able to dump excess heat from space craft and, at some point, the combination of electric devices operating within the pressure vessel and human heat output would eventually roast the people inside. We need heat to radiate outwards, and, from my understanding, it’s actually a somewhat difficult problem to solve in a vacuum. We take air and evaporative cooling for granted sometimes when on Earth and in space, where air cooling isn’t going to happen, you have to practice other methods of heat transfer.
