Greetings and brief intro

[mgritsch]

I do know about heating a piece of chalk with oxy-acetylene torch or propane torch to create an extremely bright light from the lime left behind. Hence the expression ''in the limelight'' because it was quite popular to illuminate actors on stage in the theatres.

By the way - why limestone? Where does the bright white light come from? CaO is a ionic substance, Ca should emit orange light (or does the higher temperature excite other transitions you won’t find in an ordinary flame?), not sure about the emission spectrum of oxide ions?

If it’s just the thermal emission spectrum of the solid - could any other solid that doesn’t melt at this temperature also do, like MgO etc? And why is the flame just blue and comparatively dull, shouldn’t it also exhibit a thermal emission spectrum like a white glowing hot solid?

[Glaskocher]

There is some more in the physics, than just the thermal glow of a (black) radiator. A heated substance can only emit the radation it absorbs. CaO is "white" and doesn't absorb in the thermal region. It has to emit at higher frequencies (shorter wavelength). That makes the glow very powerfull and the emission spectrum of Ca atoms is of minour importance.

One of the key words might be the "Auer Light", a gas heated source for licht, doped with rare earth elements.

[mgritsch]

A heated substance can only emit the radation it absorbs. CaO is "white" and doesn't absorb in the thermal region. It has to emit at higher frequencies (shorter wavelength). That makes the glow very powerfull and the emission spectrum of Ca atoms is of minour importance.

Sorry, I don’t really get the point… what is the mechanism of the light emission?
You say neither emission from Ca nor black body radiation… so what/how is thermal energy converted into visible light then?

Also the question remains why does a hot solid (e.g W) glow bright but a gas (e.g. Propane/O2 flame) which is even hotter is just pale blue?

[Glaskocher]

That's Kirchhoff's Law. If the substance can't absorb a given wavelength, it can not emit it as well. That means in case of CaO the emission in the (infra)red spectrum is blocked an the energy has to be released at shorter wavelength.

A black body absorbs everything. It emits at every wavelength in accordance to the absorption. If there is a substance transparent in the region it should emit radiation in accordance with Planck's Law it can not emit at those wavelangths. It has to emit at other wavelengths where the substance is less transparent. Quartzglass ist a good example for this. If it is heated little under melting temperature it emits... next to no visible light. Only the impurities, dust or scratches will emit extremely bright light.

[mgritsch]

So that means it is actually black body radiation, but because of Kirchhoff‘s laws it is shifted to shorter (ie visible) wavelengths? This sounds like a miracle energy gain, like frequency doubling in lasers