Ahhh… Same concept, but only one strip. :o
Here’s my two cents…
The flame glows because atoms in the flame are heated very hot, and emit light incandescently (just like heated metal will glow red). Just because something emits light doesn’t mean it will diffuse light, which is what is needed for the OP. Helium is almost completely transparent, but can be excited in a laser tube to emit light.
For the flame to reflect light, there would have to be particles in the flame that would diffuse the light. This is where the smoke comes in. If it is a smoky flame, then it will disperse the light, and can be illuminated with a laser.
What I don’t know is if it is these same particles that glow incandescently to make the flame visible. Will a completely smokeless fire produce an invisible flame?
I agree mostly. The flame consists of incandescent gas and if a solid material is burning there are also particles. The incandescent gas will only refract the light depending upon its index of refraction and the particles will scatter the light like shining a searchlight through the dusty atmosphere. I don’t think there is any surface on which the light can make an image.
And SnakeSpirit did that experiment.
I repeated SnakeSpirit’s experiment, and came to slightly different conclusions.
I took a 5mW red laser pointer, and aimed it at a butane lighter flame. I looked at the projection of the beam on the wall as I passed the beam through the flame. Couldn’t see a damned thing. Nothing noticeable in the flame, either.
I then lit a regular, unscented white candle. I thought maybe a flame that burns a heaver, particulate-laden fuel would be better. Butane is a clean gas, but burning wax ought to have more soot particles in it. My hypothesis appeared to have some merit. I still couldn’t see the red image of the pointer in the flame, but I watched the beam’s projection on the wall as I passed it through the flame. It did become noticeably dimmer (not a great deal, I’d estimate 5-10%). Which means that some of the light was absorbed or reflected by the flame.
I extinguished the flame, then noted the beam’s behavior as it passed through the trail of wax vapor streaming from the wick. The beam’s projection also became dimmer when the beam passed through the “smoke.” The reduction in apparent brightness was about the same, by my visual estimate. At the same time, the beam showed up in the streaming wax vapor, as you might expect a laser beam to do in a smoky region. This “image” was, of course, not terribly bright, but commensurate with the amount of brightness reduction in the wall image. My guess is that the same image was being formed in the candle flame, but was washed out by the overwhelming brightness of the flame.
My initial conclusion is that for some flames (those containing sufficient particulate), an image can be formed from projected light. However, the light source necessary to create a distingushable image would have to be many, many times brighter than the flame itself.
I have a 5mW green pointer coming to me later this week. I’ll repeat the experiment then. I plan on modifying said pointer to some illegal light output level, (say 20mW), and will compare the effects.