When light travels from one transparent material to another, it changes direction. Light bends as it travels from air into glass or water. It bends again as it leaves glass or water and re-enters the air. We call this bending of light ‘refraction’. The reason that light refracts is that it travels more slowly in glass or water than it does in air.
Refraction of light has some strange effects. It can make a stick look bent when it is lowered into water; it makes the bottom of a swimming pool seem closer than it really is; it can even make a traveller ‘see’ lakes in the desert as in a mirage.
When you look at a spoon in a glass, you see the light that the spoon reflects. Light from the spoon handle travels to your eyes in a straight line. But light from the rest of the spoon changes its speed and its direction as it passes from water to air. However, your brain assumes that the light reaching your eyes has all travelled in straight lines. You see a bent spoon which seems closer to you than it is.
The light of torch is travelling from cool air to the warmer air above the candle. Warm air is less dense than cold air, so light travels faster in it and bends, or is refracted. The same shimmering effect is produced on a hot day. Light travels faster through the hot air rising from the ground than through the cooler air above.
Mirages are caused by light bending as it passes through warm air.
Picture Credit : Google