why do mirages happen?
In: Physics
When the surface of the earth is hot, as we find in deserts, the layer of air just touching the surface of the earth would be close to the temperature of the surface. The temperature of the layers of air keep reducing as the distance between the layers and the surface of the earth increases.
The higher the temperature of air, the lower its density.
Hence, the density of air keeps reducing as we move higher up from the surface of the earth. When the density of air is lesser, its refractive index is also lesser.
Consider a ray of light from the sun along a straight line from the sun to a point at some distance in front of us, when the sun is in front of us.
If the density of air is uniform, the ray of light would travel along a straight line and touch the surface of earth a that point.
However, when the ray of light comes close to the surface of the earth, it finds that the density of air starts reducing as it is at a higher temperature. Hence the ray of light gets refracted. As it is passing from a denser medium to a rarer medium and it moves away from the normal. Moving away from the normal means coming closer to us. Thus this ray of light strikes the ground at a point closer to us as compared to the straight line path.
If the angle of incidence of the ray of light is sufficiently low and it becomes greater than the critical angle, it undergoes total internal refraction.
Then, the ray of light, instead of striking the ground starts moving in the upward direction and enters our eyes.
However, our brain tends to believe that this ray of light has travelled along a straight line path and extrapolates the path of the ray along a straight line and thus thinks that it has come from a point on the land some distance away from us.
Since the colour of the sky, as perceived by us, is blue, we then tend to believe that there is something bluish on the land some distance away.
The layers of air of different density are not static and hence the path of the rays undergoing total internal reflection vary with time.
Hence we tend to believe that, along the line of sight, the point from where the bluish light is coming is moving.
Considering all this, our brain concludes that there is a body of water some distance away. This is the mirage.
[Source](https://www.quora.com/Why-does-a-mirage-happen-in-the-desert)
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