Rainbows are sometimes abnormally shaped. The reflection of a rainbow (a reflected bow)
may be seen in a smooth water surface, and the reflection of the sun is able to generate a
rainbow in the sky. The latter is called the reflection-bow or the reflected-light rainbow.
When a rainbow is reflected, the polarization of its light hardly changes: the plane of
polarization is not reflected in the case of such a grazing reflection. Therefore, contrary to the
situation with the non-reflected bow, the direction of polarization of the reflected one no longer
follows the curvature of the bow. Hence Tangential polarization is now absent. But the rainbow
and its reflection can be extinguished almost simultaneously with a polarizing filter in the
appropriate orientation. Of course reflection of the rainbow can also be observed by putting a
polished metal plate or an ordinary mirror on the ground. Then, the direction of polarization
does follow more or less the curvature of the bow, because these materials reflect not only the
bow but also mirror the plane of polarization. In these artificial circumstances is the metal plate
in particular that additionally converts a part of the line rainbow-light into circularly polarized
light: the right part of the reflected rainbow is then left-handed circularly Polarized and vice
versa. When a horizontal glass plate or a pane is used as a reflecting surface, the polarization
is analogous to the one seen in reflection on water.
Reflection-bows are very rare and usually only sections of them are seen. The polarization of
this type of bow is tangentially directed with respect to the reflection of the sun. Hence the
polarization follows the curvature of the bow. Some deviations may occur, because the light of
the reflected sun is itself horizontally polarized, but generally they will be hardly perceptible.
The reflected dew-bow and the reflection dew-bow, which can be seen in ditches, behave in
the same way as the reflected rainbow and the reflection-bow so that by finding out whether
the polarization follows the curvature of the bow, one can decide which of the two types of bow
is seen here.
Under extremely rare circumstances extra bows sometimes appear in the sky in different
places. It usually happens near water surfaces. Some of them are reflection-bows but come
from reflection on rough water; other ones, however, can not yet be explained satisfactorily. If
you should be lucky enough to see such a reflection-bow or another abnormal rainbow take a
picture and examine its polarization. Does the polarization direction follow the shape of the
bow or not? Is the degree of polarization higher or lower than that of the rainbow itself?
Observation of the polarization of this kind of phenomenon may hold the key to the
explanation of such remarkable and rare bows.