This is due to continuous change in the refractive index of air brought about by random motion of smoke and temperature fluctuations near the fire place.
- The air rises and becomes hotter as it passes over the hot surface. Space is occupied by cool air.
- Hot air has a lower refractive index than cooler air.
- As a result, the physical state of the medium varies.
- The light tends to come from different directions due to the medium’s changing Refractive Index (RI).
- As a consequence, the apparent location of objects fluctuates.