Rainbows and Supernumerary Arcs
| Date: | Aug.,1989
| Instrument: | f=135mm
|
| Place: | BC/Canada
| Observer: | S. Kohle
|
© Copyright by the observers
The image above shows a complex rainbow phenomenon including the primary and
the secondary bow, as well as supernumerary arcs, which are located immediately
below the blue edge of the primary bow.
Both, the primary and the secondary
bow with its inverted colour-sequence, can be understood with classical optics,
i.e. deflection and refraction of sunrays through water droplets. The rays
which experience one deflection inside the droplet form the primary rainbow.
Those which are deflected twice (which amounts only to about 10 percent of the
total light) form the fainter secondary bow. The rays forming the primary bow
have a minimum deviation of 138 - 140 degrees from their initial direction,
depending on their wavelength: blue light is refracted more than red light.
Measured from the virtual center of the rainbow the observer sees the blue edge
of the primary bow on its inner side. For the explanation of the
supernumerary arcs, the principals of classical optics fail. The occurance of
minima and maxima in the light distribution can be explained by interfering
rays which experience a difference in the length of their paths while passing
through the water droplet. Those rays which differ by half of their wavelengths
interfer destructively and lead to the minima of light between the
supernumerary arcs. However, also effects of polarization of deflected rays,
sizes and oscillations of the water droplets have to be taken into account to
properly model the supernumerary arcs.
More detailed information about rainbows can be found
here.
See also:
M.G.J. Minnaert: "Light and Color in the Outdoors", Springer-Verlag
Nussenzveig H. M.: "The Theory of the Rainbow" in:
Light from the Sky, Scientific American, 1977
