Why does
the light that is dispersed by the rainbow forms a bow? The answer
involves the use of some simple geometry. The first point is that a
rainbow is not a two-dimensional arc; it is in fact a three
dimensional cone with the apex at the observer's eye. The reason that
it appears to be only a two dimensional flat object isbecause there
is no evidence of distance. All the drops, which disperse
lighttowards the observer to form the rainbow, lie in the shape of a
cone withmany different layers. The outside layers disperse the red
component, thelayer below that - the orange component, and the
layer below that -the yellow component, etc.
Consider the path of a ray
of monochromatic light through a single spherical raindrop. Imagine
how light is refracted as it enters the raindrop, then how it is
reflected by the internal, curved, mirror-like surface of the
raindrop, and finally how it is refracted as it emerges from the
drop. If we then apply the results for a single raindrop to a whole
collection in the sky, we can visualize the shape of the bow.
Figure 3.
Source:
http://www.unidata.ucar.edu/staff/blynds/rnbw4.gif
Also consider only the dispersion of the red component of the
light. It has been shown previously that the red component is seen
when the angle between the incident rays and dispersed rays make an
angle of 42 degrees. Of course beams are dispersed at 42 degrees from
drops all over the sky in all directions up, down, left, and right.
However, only red light, which reached the observer eye, comes from
the water drops which are on the cone with sideto axis angle of 42
degrees. If the observer's eye is at the apex of the coneas shown in
Figure 3, then to see the violet part of the bow we would needto look
at 40 degree to the conical axis. Therefore, the cone that
producesthe violet component comes from a cone inside that of the
cone which producedthe red part. The same argument can be made for
all the other colors, andbecause different colors are formed by
different cones, when they are viewedfrom the apex they appear in the
form of a bow.
It is interesting to note that because only the drops on the cone
with apex at the observer's eye are responsible for the formation of
the rainbow, which an observers sees, then an important implication
is that each person looking at a different rainbow. That leads
to another interesting fact that both of the observer's eyes actually
see a different rainbow, as they are both at the apex of different
cones.