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[Music]
>> Child: Hey, Dad, sometimes it seems that there is water ahead on the road. But when
you get closer, it’s not there anymore.
>> Dad: Like in this picture?
>> Child: Yes, that's it.
>> Dad: This is a mirage. In your opinion, does it happen more when it's cold or when
it's warm?
>> Child: When it's warm.
>> Dad: In any case, I have only seen this phenomenon when it was hot. This can be annoying
in the desert, if you think that there is a puddle of water when it is only the reflection
of the sky...
>> Child: And when it's cold?
>> Dad: You can also have a mirage phenomenon, but in a different way. For example, in cold
weather on the sea, it is possible that you see a boat that normally should not be seen,
because the light rays that go toward the sky are deflected downward.
[drawing]
>> This is the Earth with the sea. I draw a green boat and a yellow boat. Normally,
the yellow boat should not see the green one. But because the light rays are "deflected"
downward, he can see it nonetheless. Actually, it seems that the sailors of the
Titanic had sent lifeboats to another boat they thought to be closer. They saw its lights,
when normally they shouldn’t have.
>> Child: And for the “water” on the road then?
>> Dad: Before, I wrongly thought that the light grazing the road bounced like a tennis
ball, and that's what caused the mirror effect ... or water puddle if you prefer.
>> Dad: Later, I learned that it was due to air layers with different temperatures.
>> Child: How’s that?
>> Dad: What is the colour of the tarmac?
>> Child: Black.
>> Dad: If you touch a white metal table and another one that is black, both under the
hot sun, which one is the hottest?
>> Child: The black one.
>> Dad: Indeed, the black one absorbs more light, which converts into heat. Moreover,
a black object also releases more heat. Here, the black block underneath is the tarmac.
When it is hot, the temperature of the tarmac rises. The tarmac then warms up the air layer
lying just above it. When a light ray goes through materials with
different densities, for example air to water or vice versa, it changes its direction. This
is called the refraction. You can see it when a straw is immersed in
water. You have the impression that it is broken.
>> Child: Yes, but hot and cold air, they are the same material?
>> Dad: Sure, but with different temperatures, so with different densities.
>> Child: What happens then with the beam of light?
>> Dad: Actually, you have to imagine that there are hundreds of superimposed layers
with different temperatures. In warm weather, the light beam is deflected more and more,
up to an inflection point beyond which it goes “upward”.
This is why you can see a piece of sky or forest on the road.
>> Child: And a mirror, how does it work?
>> Dad: That's another story ...