What is mirage or optical illusion?

What is mirage or optical illusion?

PROF WAQAR HUSSAIN

          

                

          Mirage or optical illusion is a naturally occurring optical phenomenon in which observer sees water and 

inverted images of distant object that disappear on reaching near the spot.In contrast to hallucination, it is real phenomenon which can be captured in camera.Meaning of word mirage is " to look at".  In order to understand the phenomenon of mirage, first have a clear concept of total internal reflection. When, a beam of light passes from a denser to a rare medium, refracted ray bends away from the normal and the  

angle of refraction is greater than the corresponding angle of incidence. As the angle of incidence increases, the angle of refraction also increases till for a certain value of angle of incidence, the corresponding angle of refraction equal to 90⁰ .The angle of incidence for which the angle of refraction is 90⁰ is called the critical angle.

           When the value of angle of incidence becomes greater than the critical angle, the ray of light no longer suffers refraction but the whole of it, is reflected back in the denser medium. Such a reflection of light is called total internal reflection.

            

       Mirage is good example of total internal reflection. In desert during the hot day time the travelers often see distant pool of water and inverted image of trees in it.  On reaching this  

spot they see nothing but hot sand. This is an optical illusion and is known as mirage. Mirage is generally seen on road surfaces in hot season or in hot desert.

     In desert sand becomes very hot during the day time and the air, in contact with it, is heated and becomes rarer as compared to the layers of air above it. Thus as we go up, the layers of air go on becoming denser.

       The rays of light travelling downward from the top of a tree pass continuously from denser to rarer layers of air and are thus refracted away from the normal. Finally these rays meet a surface of a layer of air with angle of incidence greater than the critical angle and suffer total internal reflection. These rays are thus directed upward. If these reflected rays enter the eye of a traveler, an inverted image of the tree is seen and the hot sand appears like a lake of water. However this phenomenon is called an optical illusion.