Friday, May 30, 2014

Seeing heat: Science along the way

The other night we were cooking out when my son noticed that the air above the grill "looked like water."  You may have seen the same phenomenon on the road on a hot day.  Why can we see heat waves?

Why can we see anything?  

When electromagnetic (EM for short) rays bounce into objects, some are absorbed and others are reflected, depending on their wavelength. (You could say also say frequency, its inverse.)   Rays with wave lengths between 380 nanometers and 750 nanometers in length are received by our eyes and interpreted as colors. 

 If an eye receives all the possible frequencies equally from an object, the brain interprets the color as white. If the eye receives no electromagnetic waves between 380 nanometers and 750 nanometers from an object, the brain perceives the object as black.  So, on the above picture, we can see EM rays of around 530 nm , reflected from the green tree.

The EM waves we usually see come to us through air that is moving pretty slow, at least compared to EM waves which travel at 3 x 10^8  m/s in a vacuum where nothing is in their way.  As the air molecules gain heat energy, they start to move faster.  In fact, an air molecule may be moving so fast that it carries a little piece of the light wave with it as it moves up.  Think of an elevator for light. Our eyes perceive that light wave at a slightly higher spot than where it originated.

The next piece of the light wave may not catch an air molecule "elevator" and put eye perceives it at the original place.  These back to back changes in perception are what make the "watery effect".

So why does water look that way?

Molecules of (liquid) water move much more slowly than air molecules.  Why does it produce the same effect? Hint: The light changes as well.

Take the weekend to ponder.  I will keep an eye on the comments, so feel free to ask for more hints.  I will post the solution on Monday.

This post by Christy McGuire first appeared on WisdomKnowledgeJoy.

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