Why a flame barely blocks light
A shadow forms when an object prevents light from reaching a surface. A book does this well: its solid material absorbs or reflects much of the light that hits it. A flame, by contrast, is mostly hot gas. The gas is thin enough for much of the light from another source to pass through, so the flame may cast only a faint shadow—or none that is easy to see.
Brightness is not the same as opacity
A flame can look brilliant because chemical reactions in it produce light. But emitting light and blocking light are different properties. A lamp glows without necessarily casting a strong shadow of its own; a flame can glow for the same basic reason while remaining relatively transparent to light coming from elsewhere.
The flame is not perfectly transparent. In many flames, tiny particles of soot absorb and scatter some incoming light. Those particles can make a shadow more noticeable, especially where they are concentrated. A clean-burning blue flame, with little visible soot, may block less light than a yellow, sooty one.
The air around the flame can distort the shadow
A flame heats the air around it. Hot air is less dense than cooler air, and light changes direction slightly as it passes through layers of different density. This bending, called refraction, can make the background appear to ripple or shift rather than producing a crisp, dark outline.
The result depends on the setup: the flame’s soot content, the brightness and position of the other light source, and the surface where the shadow would fall all matter. Put a candle between a strong lamp and a wall, and the soot and disturbed air may show up as a weak, wavering shadow. The flame is visible because it shines—not because it behaves like an opaque object.
