• Question: Can a fire have a shadow?

    Asked by anon-202048 to Sophia on 4 Mar 2019.
    • Photo: Sophia Pells

      Sophia Pells answered on 4 Mar 2019:

      Shadows are caused when an object blocks light from reaching somewhere. Fire is a source of light so it can’t have a shadow.

Comments

  • Photo: George

    George commented on :

    Great question by the way. I think it is possible to at least get a weak shadowing effect. Let’s understand what a shadow is. A shadow is observed if there is some contrast in light intensity in a particular place. A shadow is seen if the variation in light intensity is large enough for our eyes and brain to process! Now it is true that lit match wouldn’t cast a shadow onto itself as it is the source of the light, but what if we shine an intensely bright torch at the lit match? A torch that is much much much brighter than the flame. Then we might expect that the flame might act to hinder some of the light and generate some light intensity contrast and therefore a shadow.

    In this case, we can get something quite interesting because of two effect called absorption and refraction (bending) of light. Absorption is important because fires generally contain small particles of carbon called soot and these particles will act to scatter and block light from passing the flame. Therefore, we already expect to have a slightly lower light intensity behind the flame, as the very intense light from the torch is blocked. The more complex effect is the bending of light, which scientists call refraction.

    **HARD**
    Refraction occurs if light passes through something that has a varying density. Well, a flame acts to heats up its surroundings and this causes the air around the flame to expand. This expanded air is less dense than the cooler surrounding air and therefore it is expected that light will refract at the edges of the flame. This is exactly why light appears to bend when it moves from air to water, because water is much more dense than air and so the (phase) speed of light changes. Only in this case, the difference in density between the expanded air and cooler air is much much smaller.

    In short, it is expected that light from the torch will also be forced around the lit match flame by this process of refraction and therefore a weak shadow should be seen. But, its still a very weak shadow because this absorption and refraction effect are quite small!

  • Photo: Andy

    Andy commented on :

    An excellent question! I reckon it can, although in non-obvious ways.

    For example, flames are made of (glowing) stuff. But it’s still stuff, and can absorb as well as emitting. If you took a small fire and shone a huge, powerful spotlight at it, I wouldn’t be surprised to find the bit behind the fire is actually less bright that the places where the spotlight was unimpeded. It wouldn’t be *dark* exactly, but less dazzlingly bright.

    You could also look in different wavelengths of light: fires tend to emit a lot in the red end of the visible spectrum and in the infra-red (which is kind of the point of fires: to get some radiative heat transfer going). If you were to shine a blue/ultraviolet light at a fire, and try to measure that light on the other side, there’s a good chance it’d show a shadowing effect.

    All this is speculation, but with a bit of science intuition behind it. The awesome thing is that you can now go and measure it, and see if your idea about how the world works is actually true or not!

    Also, what George said 🙂