Science Questions

Why does the road surface appear to shimmer when itís hot?

Sun, 1st Jun 2008

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Question

Fran asked:

Why does the road surface appear to shimmer when itís hot?

Answer

When air is hot it expands and light will go through hot air slightly faster than cold air. When light changes its speed at an angle it will bend like when it goes into glass or something.  You get lots of turbulence with air moving up which light will go through faster. So you get light bending in all sorts of directions which change as the air moves and you get this turbulence effect.

The same effect makes stars twinkle, as you get turbulence up in the atmosphere which bends the light coming from stars which makes them twinkle on and off.  Sometimes they come towards you, sometimes they go the other way.

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Fran asked the Naked Scientists: Sometimes on a hot summer day you can see "heat" radiating from really hot surfaces it sort of shimmers. Since you can't see air move, what exactly are we seeing? thanks and I love your show What do you think? Fran, Sun, 1st Jun 2008

The surface is hotter than the air. This means the air heats up & consequently rises (air - the same as all gases - expands as it is heated, causing the pressure to drop to a level lower than the air above it & that makes it rise). The rising air plays havoc with lightwaves & the shimmering you see is due to lightwaves being distorted.

Even a very short distance above the surface the air is cooler. Hotter air rising into it causes heat transference and resultant turbulence and this can also add to the effect.

I dare say 1 of our physics whizzos can explain the mechanism of heat transference better than me so I shall leave it to them. DoctorBeaver, Sun, 1st Jun 2008

Hi Fran

this is the "mirage" effect and it's caused by light altering its speed as it passes through patches of more and less dense air. Contrary to what most people think, the speed of light is not fixed and will change according to the substance through which it passes. It travels fastest in a vacuum, slightly slower in air and slower still in water. As a rule of thumb, the denser the medium  (e.g. water is denser than air) the slower light will travel in it.

But when light makes this transition, from one medium to another and so changes its speed, this also causes its path to bend slightly, which is known as refraction.

So when you look at a patch of road in the distance and there appears to be a lake hovering over the road surface, what has happened is that heat from the road surface has warmed the air above the road, making it less dense. Light hitting that patch of air speeds up and bends as it does so. Now, instead of looking at the road through that patch of air, you are actually seeing light rays that have come from the sky, so they look brighter, and bluer.

Exactly the same process makes stars appear to twinkle in the night sky as the light from them passes through denser and thinner regions of the Earth's atmosphere on it's way to reach you. If you saw the same star from space, however, it wouldn't twinkle at all.

Chris chris, Sun, 1st Jun 2008

Show off!  DoctorBeaver, Sun, 1st Jun 2008


Sorry about this Dr B but-
Convection:
The air is heated up and expands - pushing outwards against the surrounding air. As it is now less dense, it is pushed upwards by the more dense, cool, air around it (floats). This, of course, give the 'effect' of hot air rising but it is actually 'pushed up' because "there is no such thing as suck". Fluids only move when  pushed. (Except when gravity pulls, natch)
God, I'm such a pedant! lyner, Sun, 1st Jun 2008

OI... I'm the pedant around here! DoctorBeaver, Sun, 1st Jun 2008

It really does look as though I am gunning for you at the mo.
It will pass.
Look upon me as a sort of Science - obsessed Robin Hood.
I Sher Would like to get things right. lyner, Sun, 1st Jun 2008



NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO! DoctorBeaver, Sun, 1st Jun 2008

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