Can a fire have a shadow? (2024)

Category: Physics Published: December 1, 2015

Yes, you can form the shadow of a fire, but perhaps not for the reason that you are thinking. 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. With this concept in mind, in order for a fire to have a shadow, the fire needs to somehow block or redirect part of another beam of light.

Can a fire have a shadow? (1)

Fires can have noticeable shadows if the conditions are right. Public Domain Image, source: US Fish and Wildlife Services.

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.

Topics: fire, flame, refraction, shadow

As a physics enthusiast with a deep understanding of the topic, I'll delve into the concepts presented in the article about forming shadows with fire. The article discusses the formation of shadows and explores why fires can indeed cast shadows under certain conditions. Let me break down the key concepts used in the article:

  1. Formation of Shadows: The fundamental idea behind shadows is the blocking or redirection of light. When an object intercepts a light beam, it creates a region with less light, known as a shadow. The shape of the shadow corresponds to the blocking object. Shadows are not exclusive to solid objects; in certain conditions, even fire can cast a shadow.

  2. Components of a Traditional Flame: Traditional hydrocarbon flames consist of various components:

    • Hydrocarbon fuel molecules
    • Oxygen molecules undergoing combustion
    • Soot or smoke (solid bits of half-burnt fuel and impurities)
    • Carbon dioxide and water vapor produced by burning
    • Light
    • Hot air
  3. Light Emission in a Flame: The light observed in a flame primarily comes from the solid airborne soot heating up and glowing through incandescence. The interface between the hot air in the flame and the colder surrounding air causes light to bend away through refraction, similar to the effect of a lens focusing light.

  4. Refraction and Shadows: Refraction, the bending of light at the interface of different materials, plays a crucial role. The hot air in a flame, due to its temperature difference with the surrounding air, causes light to deflect. This effect contributes to the creation of shadows. The article notes that shadows created by hot air may exhibit dancing ripples.

  5. Shadow Visibility: To observe the shadow of a fire, the light beam passing the fire (e.g., sunlight) must be sufficiently bright compared to the light emitted by the fire. If the fire's light is stronger, it will fill in any dim regions created by the blocking or redirection of light. The size, temperature, and soot content of the flame also influence the visibility of its shadow.

  6. Light Interaction on a Fundamental Level: The article emphasizes that the shadow is not formed because one beam of light scatters off the light in the flame. On a fundamental level, light beams, consisting of photons (bosons), do not directly interact with each other. Photons lack electric charge and magnetic moment, preventing direct influence between different beams of light.

  7. Exotic Effects and Light Interaction: While one light beam cannot directly influence another, indirect effects, such as deflecting one light beam by changing the material both travel through, are possible. However, the article notes that such effects are not present in traditional flames.

In summary, the formation of shadows by fire involves a combination of factors, including the components of a flame, refraction of light, and the interplay between the fire's light and external light sources. The explanation goes beyond conventional observations and delves into the fundamental nature of light particles and their interactions.

Can a fire have a shadow? (2024)
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