Again, the light behind the flame must be brighter than the flame. Your image the flame is brighter. Of course if the flame is the source of the brightest light, it will not create a shadow. Also a more sooty flame will create more viewable flame.
Here is a flame with no shadow because it under regular lighting. Followed by the same flame viewed under light from a sodium lamp (the light is brighter than the light from the flame and casts a shadow)
This post was edited by my2cents at March 18, 2024 12:41 PM MDT