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.
“Halogen” bulbs are just incandescent bulbs filled with inert gas and a small amount of bromine or iodine. Very interesting chemistry https://en.wikipedia.org/wiki/Halogen_lamp
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.)
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.
Same way sun heats the earth
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?
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.
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.
Maybe they are thinking of how mugs and thermoses can be labeled “vacuum sealed,” and that the marketing implies that the vacuum between the walls insulates the outer wall (where the hand touches) against the heat.
And those are usually made out of some metal, that’ll do pretty good reflecting radiation heat transfer
for better insulation you can put more reflecting layers inside. i’ve heard of insulation for liquid helium pipe, it used thousands of layers of aluminized mylar between two tubes in vacuum. it’s one barrier between 4.2K and room temperature, and it works good enough to be used in helium manufacturing plant
You ever seen those vacuum ovens too? Kinda doing the opposite, only letting radiation in, but from just solar radiation I’ve heard in sub zero temps they can get to a constant 500 F. Wonder what one would be capable of with some one way mirror type refraction to keep all that shit in
yes but no reflective surfaces involved this time, at least not for insulation. compare that to vacuum tube water heater, it has a heat pipe with a fin, painted black, which is in turn insulated with a two walled evacuated glass tube. it’s there just to stop convection from carrying heat away. i guess something very similar is going on there
It does do that though. My vacuum thermos keeps things hot or cold for like 6 hours
More then you ever thought you needed to know about lightbulbs! (Including the fact that inconsistent light bulbs are not vacuumes, they just have no oxygen, so they heat up via normal connection)
How did I know before even clicking that this would be a Technology Connections video
I literally watched this video about 10 hours ago. He’s the Internet’s favorite Chicagoan.
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.
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.
