What causes the swively lines above a fire or heater?
(Lansing State Journal, November 16, 1994)


What you are seeing are air disturbances caused by heat.  Normally we cannot see air because its molecules are too dilute.  But we can perceive differences in air density if they are extreme enough.  The density of air depends largely on temperature and pressure.  Heat will increase air temperature, which in turn affects ai pressure, resulting in a change in air density.

When light passes through a material, like air or water, it slows down.  This slowing causes the light to bend or refract, at a particular angle.  (To see this, put a spoon into a clear glass of water.  The spoon will appear bent at the boundary between air and water.)  The exact angle depends on the material and its density.  So, when air density changes, light passing through it bends.

Now about a fire or heater, some groups of air molecules become hotter than others nearby.  So, different regions above the heat source have different air densities.  As light passes through these different regions, it bends in different ways.  You can see the bending light waves as those swively lined air disturbances.  Because the molecules are moving quickly, the patterns of lines keep changing.

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