The Shadow of Fire

Today’s question is something that is mostly debated by my physics friends. Let’s get straight to the point:

Does Fire Cast a Shadow?

Not if it is the only light source present in a room. It can hardly be noticed most of the times. But there is a hint of a shadow in a candle flame.

One can see “something” in the background wall of the above image. The light shadow is not really a typical shadow of the flame though. Let’s delve a bit deeper into the darkness.
An object casts a shadow when it blocks a ray of light. Most objects form shadows by absorbing light that is falling on them. The image below depicts how a spherical object for instance casts a shadow on a flat screen.

One can see that unlike a normal shadow, the shadow of the flame above has two distinct parts (or properties). The inner part is like a normal shadow and a bit dark compared to the surroundings. This is a traditional shadow cast by the soot and unburned residues present in the flame center.
The other part is the lightly darkened part outside of it surrounded by the bright outer ring of light. The flame is transparent and doesn’t absorb any light passing through it. However, the gas in the flame is hot and has lower density compared to the surrounding air. This leads to refraction (bending of light) as it passes through the flame. The refraction causes a deflection of light towards the boundary. This leads to the lightly darkened part of the shadow.