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Fires can have shadows too!

Q: Why does fire have no shadow?

Krishna: To understand my answer, first watch this video

Why does fire have no shadow?

Who says so?  Flame Fires Can Have a Shadow! Here is the Picture proof!

Image credit: Robert Frost

And video proof:

How are shadows formed? When light is obstructed, a shadow is generated. But we can see that when a bright flame is crossed by a weak light source, no shadow is cast at all. Because a flame is always a source of light, it will not cast a shadow by another source of light going through it. Essentially, one light beam can not interact directly with another.
But a fire flame can have a shadow because the flames also include hot air, soot, and combustion products, which all can obstruct light, you will see fires cast shadows if you illuminate a flame with a stronger beam of light source. In a light beam, the shadow zone is the area where there is lower light than the majority of the beam.
A flame is also a source of light. It will not cast a shadow. This is a relative statement , though. Flame may project a shadow in a powerful light beam because flames contain hot air, soot, and combustion products.
Shadows cannot exist in anything that is fully transparent. When the light beam is obstructed, shadows form, and the shadow area is the part of the light beam that has less light than the rest of the beam.
Light sources cannot cast shadows since they all emit light waves that move away from them. However, in rare cases, light sources may project a slight shadow under a powerful light beam.
A shadow is formed any time part of a light beam is blocked or redirected. The shadow region is the region in the light beam where there is less light than in the rest of the beam. This dimmer region tends to take on the shape of the object that is blocking or redirecting some of the light, so we tend to think of a shadow as something that is cast or created by the intercepting object (2).
In order for a fire to have a shadow, the fire needs to somehow block or redirect part of another beam of light.
A traditional flame can block or redirect light just fine for the simple reason that a traditional flame is more than just a pillar of light. A traditional hydrocarbon flame contains several components: hydrocarbon fuel molecules and oxygen molecules that are in the process of burning, little solid bits of half-burnt fuel and impurities (called soot or smoke), carbon dioxide and water vapor produced by the burning, light, and hot air. The light that you see in the flame is mostly created by the solid bits of airborne soot heating up so much that they glow via regular incandescence. The interface between the hot air in the flame and the colder surrounding air tends to bend light away from its forward-propagating direction. This deflection of light at the interface of different materials is called refraction. It is the same effect that enables a lens to focus light. Therefore, for the simple reason that a flame contains hot air, it is able to deflect away some of the light in a light beam and cast its own shadow. Hot air tends to rise turbulently. For this reason, shadows created by hot air tend to look like a bunch of dancing ripples. Also, the soot in a flame can absorb light and can therefore also contribute to the creation of a shadow of the flame.

To actually notice the shadow of a fire, the light beam that is going past the fire (e.g. sunlight) must be about as bright or brighter than the light created by the fire itself. Otherwise, the light created by the fire, which spreads out in all directions, will overpower and fill in any dim region created in the other beam of light. For example, pointing a weak flashlight at a roaring campfire will not enable you to see the shadow of the fire. Also, the smaller and cooler a flame is, and the less soot it has, the less it absorbs and redirects light, and therefore the dimmer its shadow will be. Depending on your particular setup, you may or may not be able to see the shadow of a flame with your naked eyes. For best results, you should use a bright light beam, such as direct sunlight, and a fire with lots of heat and soot.

Note that fire can have a shadow not because the incoming light beam scatters off the light in the flame. On the fundamental level, one beam of light cannot directly interact with another beam of light . Light beams never directly bounce off each other, absorb each other, or deflect each other. This is because light consists of quantum particles called photons which are by nature bosons. All bosons are able to overlap with each other, pass through each other, and occupy the exact same state at the exact same location. This is also because photons carry neither electric charge nor magnetic moment. Electromagnetic fields, such as those which constitute light, can only interact with objects that carry electric charge or a magnetic moment. Without any charge or magnetic moment to interact with, one bit of light cannot directly influence in any way another bit of light. Note that one light beam can indirectly deflect another light beam by changing the material that both beams are traveling through, or through more exotic effects, but such effects are not present in traditional flames. Fires can have shadows because they contain hot air and soot, and not because they contain light (1).

Footnotes:

Views: 174

Replies to This Discussion

Permalink Reply by Dr. Krishna Kumari Challa on March 25, 2022 at 9:27am

74

Because it is a light source, not a thing that blocks light.

That statement, however, is a relative thing. If a dim source is in front of a bright source, it often CAN cast a shadow. This is what happens in stars called eclipsing binaries. 

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