Question

Why does an object appears to be wavy when seen throught hot air over the fire

Answer 1

When light passes through different mediums it refracts.

e.g. when a pen is placed inside water it looks bent.

Air hotter than its surroundings also refracts light slightly. So above a fire the hot air constantly change, so where the light is bent it too changes. This makes the area above the fire appear wavy or to be slightly moving.