Mirage ~ An optical illusion

Have you ever been to a desert? Yes?Then you definitely encountered this phenomena in your journey. If no? You do drive car on a hot summer day right? Ever noticed the roads that appears to be wet? Ah let's pull this topic today!

In a desert you may noticed a lake while searching for water and you be like Yayy!!! But then you find that it was not really a lake instead it was a Mirage! So the questiom is what is a mirage and how does it forms?

Well let me tell you that Mirage is an optical illusion formed due to Total internal reflection. 

Again now what is total internal reflection?

When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection. But in case of total internal reflection there is no transmission of light takes place and there is no loss of intensity.

Essential condition for total internal reflection

1)Light should travel from a denser medium to a rarer medium.

2)Angle of incidence in denser medium should be greater than the critical angle for the pair of media in contact.

{Critical angle for a pair of media in contact as the angle of incidence in the denser medium corresponding to which angle of refraction in the rarer medium is 90°}

How does a mirage forms?

Till now we came to know that mirage is an optical illusion which occurs usually in deserts on hot summer days. As for example object such as a tree appears to be inverted, as if the tree is on the bank of a pond of water.

On hot summer days, the air near the ground becomes hotter than the air at higher levels. The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with hight. As a result, light from a tall object such as a tree passes through a medium whose refractive index is decreases towards the ground. Thus a ray of light from such an object successively bends away from the normal and undergoes total internal refelction. If the angle of incidence for the air near the ground exceeds the critical angle. This is shown in the figure.(b) . To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage. You nay have noticed that while moving in a bus or a car during hot summer day, a distant patch of road, especially on a highway, appears to be wet. But you don not find any evidence of wetness when you reach the spot. This is also due to mirage.

Picture sources:

1) www.nullarborroadhouse.com

2) www.planet-science.com

Reference:

1)Fundamental Physics by K.L. Gogia and K.L Gomber

2)NCERT textbook 

Thanx for going through my post and have a great day!