What Is Inferior Mirage And Superior Mirage
Mirage or Inferior Mirage
Definition: It is an optical illusion, seen in deserts at summer noon, due to which an inverted image of a distant tree is seen formed in hot sand below it, as if formed in water. Actually there is no water anywhere.
Fig. Mirage in hot desert–Inverted image of tree in hot sand
Explanation: It is due to total internal reflection. At summer noon, in desert, sand becomes hot. The air in its contact becomes very hot and hence rarer. As we move up, air becomes less and less hot, hence less and less rarer. The air can be divided into layers of different optical density (fig.).
For rays (like number 1 ray), incident at small angle at upper most layer, the angle of incidence on lowest layer may not be more than critical angle. These rays are all absorbed by sand.
The ray no. 2 starting from tree top and making a bigger angle since beginning, reaches the lowest layer at bigger angle. The angle of incidence may become just more than the critical angle. The ray is totally reflected upward and outward. All rays on the right of ray no. 2 will start with still bigger angle and will have angle of incidence becoming more than critical angle from upper and upper layers. They are also totally reflected.
Looming or Superior Mirage
Definition: It is an optical illusion, seen, at sea–shore in winter evening, due to which an image of a ship is seen formed in air in sea–sky. The actual ship is nowhere visible.
Fig. Looming at cold sea–shore
Explanation: It is due to total internal reflection. In cold evening, over sea–bed sea water becomes too cold. Air layer in its contact is cold and denser. As we go up, air layers become less and less colder and hence rarer. (Fig.)
Rays from invisible ship going upward go from denser to rarer air layers. They are totally reflected downwards and received by an observer at sea–shore. The observer sees an image (virtual) of the ship hanging in the sky.