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Q & A: Mirages and the puddle effect

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Most recent answer: 10/22/2007
Q:
this is about the puddle effect... the puddle effect is when you see little odditys in the road that look like puddles it happens because the light that the sun creates heats the road and then expands the air which acts like a magnafiying glass and creates the effect... my question is if the light FROM THE SUN is bent beacuse of the light FROM THE SUN is the light bending itself? or do i have it wrong?
- James
A:
This is a very good, perceptive question! Yes, of course the optical artifacts that look like puddles on the road are due to variations in the refractive index of the air. Some bits of air (like that near the surface of a road) have different temperatures from other parts of the air and the air acts a bit like a lens, bending light from its straight path. The "puddles" are called mirages.

Of course the heating is often due to the sun (it can be due to other things like the burning of fuel). Sunlight refracts in the atmosphere just like light from any other source, and so the sunlight has two roles -- one of which is to heat surfaces and also the air, and the other is being bent by the differentially heated air to arrive in various places with various strengths.

One observable consquenece of this is that the disk of the sun is distorted out of shape as it sets. Sunlight from the setting sun must travel through many layer of the atmosphere, at differnet temperatures and pressures, and this may make the setting sun even look as if it is made up of pancakes of different thicknesses stacked on top of one another, and even produce little disconnected blobs of light.

It also is a limitation of shining very powerful laser beams from space to Earth, or from one place on Earth's surface to another. The laser beam heats the air it travels through, creating a lens which spreads the beam apart.

Tom


Just in case you were asking about sunlight DIRECTLY causing its own bending, that isn't happening. It's only via the indirect means that Tom described. mike w

(published on 10/22/2007)

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