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ICSE Class 10 Answered

How does phenomenon of mirage take place?
Asked by Kanwaranita10 | 07 Aug, 2019, 05:43: PM
answered-by-expert Expert Answer
The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index
than the cooler air. When the temperature difference is negligible at different height from ground level, we see the
things as they are.  For example, a tall tree is seen by observer without any change as shown in fig. A

On hot summer days, the air near the ground becomes hotter than the air at higher levels.
If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height.
As a result, light from a tall object such as a tree, passes through a medium whose refractive index decreases
towards the ground. Thus, a ray of light from such an object successively bends away from the normal and
undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle.
This is shown in Fig. (b). To a distant observer, the light appears to be coming from somewhere below the ground.
The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object.
Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage.
 
 
This type of mirage is especially common in hot deserts. We might have noticed that while moving in a bus or a car during a hot
summer day, a distant patch of road, especially on a highway, appears to be wet. But, we do not find any evidence of wetness
when we reach that spot. This is also due to mirage.
Answered by Thiyagarajan K | 08 Aug, 2019, 09:39: AM
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