Because the air neat fire gets heated.the heated air disturbe the light ray passing through the image looks swaying.the light ray gets refracted in heated air

please mark as brainlist
The Brainliest Answer!
When air is heated the the surrounding air move upward...i.e from higher temp to lower temp....hence the air above the fire gets denser.According to law of light refraction...when light moves from rarer medium to denser medium,light rays bents away from normal.Hence with fluctuation in density fluctuation of image takes place.
Hope this anwer helped.......comment if doubt persists......!!! :) 
1 5 1