Question

Explain the formation of mirage.

(OR) 

How is the mirage formed? Explain. 

(OR) 

A person walking on a road observed some water being present on the road but when he went there actually he did not find water. What is that actually formed called? Explain that process. 

(OR)

Sometimes during the hot summer at noon time on tar roads, it appears that there is water on the road, but there would really be no water. What do you call this phenomenon? Explain why it happens. 

(OR) 

Why do you see a mirage the road on a hot summer day?

Answer 1

• During hot summer day, air just above the road surface is very hot and the air at higher altitudes is cool. 
• We know that refractive index of air increases with density. 
• So, the cooler air at the top has greater refractive index than hotter air just above the road.

• Light travels faster through the thinner hot air than the denser cool air above it. 
• On hot days, the temperature decreases with height. 
• Thus the refractive index of air increases with height. 
• When the light from a tall object such as tree or from the sky passes, through a medium just above the road whose refractive index decreases towards ground, suffers refraction and takes a curved path because of total internal reflection. 
• This refracted light reaches the observer in a direction shown as in second figure. 
• This appears to the observer that the ray is reflected from ground. 
• Hence we will see water on road, which is the virtual image of sky and an inverted image of tree on the road.
• Such virtual images of distant high objects cause the optical illusion called ‘mirage’.