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First, I would like to clarify that I know why a mirage is formed, what I want to know is why is it that when a mirage is formed it appears that a pool of water is present.

Like for a palm tree in a desert, an inverted image is formed accompanied by a virtual pool of water. But where does that pool of water come from? The only object whose image is inverted is the tree isnt it?

enter image description here

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    $\begingroup$ When one encounters a tree in a desert, it is a fair assumption that there is a pool of whater nearby ;) $\endgroup$ Feb 11 at 8:38
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    $\begingroup$ @RogerVadim - you must go to different deserts than I do. What we call 'rivers' around here are what many folks would call 'trickles' or 'puddles'... $\endgroup$
    – Jon Custer
    Feb 11 at 19:41

3 Answers 3

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The reflected image of the palm tree is accompanied by the reflected image of the sky above and surrounding it. so in the reflection, you see the palm tree and the sky. There is no pool of water: the light coming towards you from the palm tree and the sky behind it is reflected up away from the ground by a hot layer of air right next to the ground, which acts like a mirror and inverts the image of the palm tree.

Note that at a shallow angle close to sea level, the sea far from you also acts like a mirror, reflecting the blue of the sky, when the surface of the sea is calm and nearly flat. The reflections swirl around a bit because of small waves that "bend" the mirror surface around.

Because there are weak air currents next to the hot ground in the mirage case, the reflections from distant objects get swirled around in a manner that looks like the surface of a pond with small waves in its water.

These effects are easy to demonstrate if you put a table tennis table out in the sun and at one end, you install miniature palm trees. Somewhere near the middle of the table you place a large flat mirror and next to it a very shallow plate with a thin layer of water in it. Then go to the opposite end of the table, get down on your knees, and put your eye right next to the edge of the table and look towards the little palm trees, moving your head until the reflectors are in your line of sight.

With care you can do this experiment with a big flat plate of hot metal in place of the mirror but you might start the table on fire.

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  • $\begingroup$ Not necessarily hot metal: a normal-temperature mercury would work (though it has another hazard :)). But, there are other metals liquid at almost-room temperature, like gallium, or at a bit higher (still far from staring a fire) like Rose's alloy. $\endgroup$
    – Ruslan
    Feb 11 at 11:06
  • $\begingroup$ @Ruslan, I mentioned hot metal because it would create the thermal gradient that bends the light. Hope you are doing well. best regards, NN $\endgroup$ Feb 11 at 17:03
  • $\begingroup$ Hmm, won't then a metallic surface affect our ability to distinguish a mirage from usual reflection? $\endgroup$
    – Ruslan
    Feb 11 at 17:05
  • $\begingroup$ @Ruslan, no. it is a physically correct way to simulate the mirage formation process. $\endgroup$ Feb 11 at 17:13
  • $\begingroup$ I mean, a metallic surface is intrinsically reflective. As you heat it up, how do you know at which point you got a mirage instead of the normal reflection? Will the reflected object shift, or will it simply become wavy, or something else? $\endgroup$
    – Ruslan
    Feb 11 at 17:17
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The water doesn't appear, we generally consider reflections on the ground to be some water body, as we don't generally have shiny surfaces on roads or deserts. So the reflection is often mistaken for a body of water shining and reflecting the surroundings in the sun.

The reason for the reflection is clear to you I guess from reading your question, so yeah there you go.

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Broadly from two possibilities.

One is that there really is a reflection of water; unlikely, but not impossible.

The other is that what somehow looks like "water" is in fact nothing but the rippling or waving effect of the interaction of layers of air with different properties; most obviously, different temperatures and possibly also, different humidities, among other characteristics.

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