Physics Photo of the Week

September 28, 2007

Loons and Mirage
Common loons have made a remarkable comeback in northern US lakes and Canada in the past 30 years.  At Willoughby Lake, Vermont loons are often seen socializing and "showing off" in groups.

Of special physics interest in this photo is a subtle mirage effect seen with the rocks on the distant shore, which is part of Willoughby State Forest.  Examine the close-up of the shoreline rocks in the image below.  Notice that each rock shows a mirror image reflected immediately below it, as if the lake were acting as a perfect mirror.  Those reflections are caused by a mirage, not the relatively smooth lake water.  The warm air layer immediately above the lake water causes this mirage.



The drawing below illustrates the physics of a mirage.  The lake mirage was observed in the early morning after a relatively cold night in the middle of summer.  Because of the high heat capacity of the water, it maintains a relatively constant temperature during the cold night, whereas the surrounding air was able to cool off.  The ambient air in the early morning was colder than the lake water.  However, the air immediately above the water surface (the lower 10 cm or so) is maintained warmer than the ambient air due to contact with the warm lake water.  The thin layer of warm air immediately above the water surface is responsible for creating the mirage.  The same effect happens on highways during warm, sunny days (see last week's PPOW).



Because warm air has a lower density, it has a lower refractive index than cold air.  This means that warm air more closely resembles a vacuum than the cold air, and light travels a little bit faster in the warm air than in the cold air.  Whenever light is leaving a dense region to enter a less dense region, glass to air, cold air to warm air, etc, it undergoes total internal reflection if the incident angle is close enough to the plane of the interface.  For glass to air, this angle is about 48 degrees to the surface of the glass.  For the very small difference in refractive index between the cold and warm air, this angle is considerably smaller than 1 degee.  Thus only grazing light from objects very close to the opposite shore get reflected by the mirage.

Strickly speaking, the mirage light is refracted, not reflected.  Instead of a sharp interface between the warm air and the cold air, there is actually a temperature gradient, where the temperature changes gradually from warm to cold.  The grazing light ray is actually bent or curved through the temperature gradient rather than a sharp reflection.  However, the overall effect is the same.  The mirage light appears as if it were totally internally reflected by the layer of warm air over the water surface.

The loons could care less....



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


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