Shouldn’t the vacuum insulate the glass from the heat of the burning filament?
Vacuum means no convection heat. It however does not mean no radiation heat. The filament radiates heat through vacuum the same way the sun does.
Additionally, theres not vacuum in buldbs, but inert gas, like argon.
There no longer is a vacuum in bulbs, but there was for a very long time.
In addition to the stuff everyone else is saying, most modern bulbs don’t have a vacuum at all.
Most modern bulbs are filled with an inert gas like argon or xenon. Usually at a lower pressure (around 70% of standard atmospheric pressure), but nowhere near a vacuum.
This has, while inert to chemical reaction, is more than capable of transferring heat.
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.
Wow, I fucking learned something today. Thank you, stranger.
very closely related:
Go see what happens if you lick a radio transmitter aerial and report back
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.)
3 forms of heat transfer
Conduction. Transfer of heat from one medium to another.
Convection. Transfer of heat within the same medium.
Radiation. Non-contact transfer of heat.
In addition to what others have said, they’re not a vacuum inside. They’re filled with 0.7 atm of argon gas. That would slow the transfer of heat, but there’d still be plenty of heat transfer through convection.
Why not 1 atm?
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.
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.
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.
