Question

why do the faces of persons sitting opposite to you around a camp fire appear to shimmer?

Answer 1

The opposite person’s face twinkles due to

The refraction of the rays of light reflected from the opposite person’s face through the hot air. Since the hot air keeps on moving the path of the refracted rays too change.
Refraction: the fact or phenomenon of light, radio waves, etc. being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density.