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All objects reflect some of the colours and then our eyes are able to see object of different colours. But when we see mirage in a desert then it seems that the sand of the desert is acting as a mirror. So what makes the sand a mirror in this case? Is this because the light reflected from an object when falls on the sand is also reflected back to our eyes from the sand ? What makes the sand to behave like a mirror? I know about total internal reflection but then what makes the sand to reflect so clear image of an object?

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Light travels very slightly faster in hot air than it does in cool air. In bright sunlight, the surface of the ground gets hotter than the air a few feet above it, which means that the air right next to the ground is hotter than it is a few feet above it. Therefore, a ray of light that is approaching the ground at a shallow angle will speed up a tiny amount as it gets close to the ground while the rest of the light ray that is a few feet above the ground does not.

This has the effect of gently bending the light beam up and away from the surface of the ground just as if it had struck a mirror lying on the ground.

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  • $\begingroup$ Nicely explained! $\endgroup$
    – Gilbert
    Jun 7 '18 at 0:47
  • $\begingroup$ I've always wondered, once the light is refracted to the point where it's parallel to the ground, and presumably then passing through air with a consistent index of refraction, what then makes it refract upwards? $\endgroup$
    – M. Enns
    Jun 7 '18 at 2:23
  • $\begingroup$ Also, if the light is refracted rather than reflected, would an image formed in this way be inverted? $\endgroup$
    – M. Enns
    Jun 7 '18 at 2:25
  • $\begingroup$ the refractive gradient is concentrated very close to the ground, which is what turns the beam upwards. the image is not inverted; you can demonstrate this with simple ray-tracing. Some years ago Scientific American magazine did an excellent article on the optics of mirages, perhaps you could look it up. In the old days, SciAm was a very good source of solid and very well-written information. $\endgroup$ Jun 7 '18 at 4:33

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