# Hot glass

1. Feb 17, 2008

### Physicsissuef

Why when sometimes touch the glass balloon around the light bulb, it is hot?

2. Feb 17, 2008

### ||spoon||

the filament of the globe is extremely hot. I suppose it is just heat transfer from this.

3. Feb 17, 2008

### Physicsissuef

Isn't just visible light released from the filament? Where the IR light comes from?

4. Feb 17, 2008

### ||spoon||

the light from the filament is caused by the random excitation of electrons due to the heating of the filament due to the current passing through it. This is why the light voming from a lightbulb is incoherent.

5. Feb 17, 2008

### Physicsissuef

There is light because of the collision of the electrons with the atoms of the filament. So that's how there is light. But where the heat come from??

6. Feb 17, 2008

### CompuChip

Ordinary light bulbs are very inefficient. In fact, only between 2 and 10% of the output is actually visible light, the rest is heat. I think in modern light bulbs the efficiency is somewhat higher, up to 30% and maybe even more. Still this means that over half of the energy is "wasted" (of course, you can calculate how much you can turn down the heating if you have a couple light bulbs burning).

Basically what happens in an ordinary light bulb is that a material (usually tungsten, aka wolfram) is heated up by pumping electricity though a very thin wire. Almost as a side effect, the wire emanates light (just like any metal does when heated, compare molten iron).

Last edited: Feb 17, 2008
7. Feb 17, 2008

### CaptainQuasar

Forget that the filament gives off light for a moment. The filament is an electrical resistor, just like the heating coil in an electrical heater. When electricity is passed through it its resistance creates heat. This heat radiates outward from the filament and increases the temperature of the bulb glass and the other components of the bulb.

8. Feb 17, 2008

### ||spoon||

what causes the electrons to be of such a high energy to release light?

Heat does. The filament has a high resistance so when a current is passed through it alot of heat is generated. This causes electrons to become excited... Moving between discrete energy levels. When they drop from a higher energy level to a lower a certain frequency of light is emitted.

9. Feb 17, 2008

### Staff: Mentor

The principle was mentioned by description, but not by name: The filament of a light bulb is a black body radiator. So it gives the entire spectrum, with a center around a frequency corresponding to its temperature. That's the reason that incandecent lights are so inefficient: they give off way more heat than visible light.

10. Feb 17, 2008

### Claude Bile

Also, glass is quite opaque beyond a wavelength of 2 microns, and therefore absorbs most of the heat being emitted by the bulb.

Claude.