Does fire have a shadow?

Unless you’re Peter Pan, you can’t lose your shadow. At least not permanently.

Shadows appear wherever light is obstructed by an object, and come in a variety of shapes and sizes. In 2012, for example, physicists caught the first view of an atomic shadow by laser illumination of a single ytterbium atom in a vacuum chamber.

And on a much larger scale, our moon blocks the Sun’s light from reaching Earth during solar eclipses. Millions of people will witness this firsthand in 2024, when a long-awaited total sweep North America.

But what happens if the obstacle myself is also a source of light – like a candle flame or a roaring campfire?

What is fire made of?

First of all, most fires are more than a source of light. A typical flame contains many: hydrocarbon and oxygen molecules that are actively burning, the carbon dioxide and water vapor produced as a result, and impurities such as soot, smoke, and half-burnt fuel.

The yellow light we usually associate with fire is actually created when tiny pieces of soot get heated enough to glow. These yellow flames are called shining and all that extra soot eventually means they aren’t getting enough oxygen to turn the fuel into carbon dioxide.

Non-luminous flames, however, are a different story. Because they have access to lots of oxygen, they burn cleaner (read: soot-free) and hotter. They also tend to burn blue rather than the traditional yellow.

Does fire have a shadow?

What does all this have to do with shadows? Well, the more visible impurities the fire contains—soot, but also perhaps aerosolized candle wax—the better the chance you’ll be able to see its shadow.

Of course, even a fire that burns cleanly has something like a shadow. This usually looks like faint, dancing ripples and is a visual representation of the super-hot gas molecules dancing around in the flame; because the molecules are much hotter than the surrounding air, they bend or diffract the light.

Photographers may recognize it as the same science that allows a camera lens to focus light.

Read more: Do atoms ever touch?

What is light anyway?

Try pointing a dim flashlight at a blazing campfire, though, and you’re unlikely to see much of a silhouette. In order for a flame (or other light source) to generate a shadow visible to the human eye, it must interfere with an even brighter lighter source – such as sunlight.

Note that each shade is no the result of the incoming light being scattered by the light produced by the fire. Physics tells us that light rays never directly interact with each other, whether that means bouncing off or absorbing each other.

Why? Photons, the tiny particles that make up light waves, can by nature overlap, pass through each other, and even occupy the same positions. They are the very definition of mind your own business.

Photons also have no electric charge and a magnetic moment, things that electromagnetic fields take advantage of to interact with objects. But without them, one ray of light can never interact with another ray of light.

In short, fire does not create a shadow, as you or I might, by blocking all light from passing through it. But it partially blocks the light – and thanks to a bunch of super-hot gas molecules, some of the light is likely to be deflected back or diffracted.

To avoid the Peter Pan effect at home, make sure you point a bright light beam or direct sunlight at a glowing flame that has a lot of soot or smoke.

Read more: How flames behave in space