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.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
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