Question

why does the image of an object around a flame looks shaky(like a few centimetres around the flame)

Answer 1

this is the "mirage" effect and it's caused by light altering its speed as it passes through patches of more and less dense air. Contrary to what most people think, the speed of light is not fixed and will change according to the substance through which it passes. It travels fastest in a vacuum, slightly slower in air and slower still in water. As a rule of thumb, the denser the medium (e.g. water is denser than air) the slower light will travel in it.

But when light makes this transition, from one medium to another and so changes its speed, this also causes its path to bend slightly, which is known as refraction.


So when you look at a flame of the candle .what has happened is that heat from the flame surface has warmed the air above the road, making it less dense. Light hitting that patch of air speeds up and bends as it does so.