Mirrors in the air: mirages in nature and in the laboratory

M Vollmer
Figure 7. Typical inferior mirage, observed on a hot summer street. Such mirages can be seen nearly every day
while driving a car, when the Sun shines for a long time on paved streets. (Photo E Tränkle, reproduced with kind
permission.)
Figure 8. Inferior mirage observed on the Wannsee in Berlin. The water had already warmed up in early spring
before cold arctic air was rushing in. As a result, a normal boat seems to look like a ‘sea daemon’. (Photo
ETränkle, reproduced with kind permission.)
on hot summer days. The formation of inferior
mirages is illustrated in figure 6.
Some examples from observations in everyday
life and nature are shown in figures 7 and 8.
The angular size of mirages is usually
about 0.5 ◦ –1 ◦ . For observations, binoculars and
cameras with telephoto lenses are used. The
mirages are often distorted and flickering due
to local density fluctuations in the air. This is
very similar to the flickering of stars at night,
seen through the atmosphere. Therefore, it
is not possible to get really ‘focused’ mirage
photographs.
Invisible objects: the vanishing line
A prominent feature of a mirage is the so-called
vanishing line of the object [2]. Light below
certain object points has no chance whatsoever of
reaching the eye of the observer, i.e. parts of the
object cannot be seen, irrespective of the direction
into which the light is scattered (see figure 9).
The concept of vanishing lines explains why
mirages often show incomplete images of objects,
which often makes it difficult to decide what is
actually being observed. Figure 10 depicts two
photographs taken close to the coast of the North
Sea in Germany.
168 P HYSICS E DUCATION March 2009