Thanks for the great question and let me
complement you on your great intuition.
are correct in guessing that the reason rainbows
are bent is due to moisture in the air. I'll try to
give you a more detailed discussion of this
amazing feature of rainbows.
consider just a single drop of water and how light
travels through it. As light enters the raindrop,
it is refracted (the path of the light is bent to
a different angle), and some of the light is
reflected by the internal, curved, mirror-like
surface of the raindrop, and finally is refracted
back out the raindrop toward the observer.
I've tried to attach a picture to help
show the following discussion. This schematic
represents the path of one light ray entering a
raindrop at point A. As the light beam enters the
surface of the rain drop, it is bent (refracted) a
little and instead of continuing to point D,
strikes the inside wall of the raindrop at point
B, where it is reflected back to point C. As it
emerges from the raindrop, it is refracted (bent)
again into the direction E. The angle created at
point D is 42 degrees.
The ray drawn here is
significant because it represents the ray that has
the smallest angle of deviation of all the rays
that can enter the raindrop. This means that much
of the sunlight that is refracted and reflected
through the raindrop is focused along this path.
This is known as the rainbow ray.
various colors that make up white light all have
slightly different wavelengths, each color becomes
slightly separated from the others as the light
ray is refracted and reflected. So, rays that
strike the raindrop at this angle of 42 degrees
will tend to form a concentrated, strong beam in
which the colors will be widely separated. This
creates the color bands in the rainbow, with blue
along the inner portion of the bow, and red on the
outside edge of the bow.
Since the raindrop
is circular, the reflection it creates is also
circular. We don't see a full circular rainbow
however, because the earth gets in the way. The
lower the sun is to the horizon, the more of the
circle we see. At sunset, we would see a full
semi-circle of the rainbow with the top of the
arch 42 degrees above the horizon. The higher the
sun is in the sky, the smaller the arch of the
rainbow above the horizon. If the sun is more than
42 degrees above the horizon, no rainbow will be
1. You need to be
standing with the sun to your back and the rain in
front of you.
2. The sun needs to be less than
42 degrees- above the horizon.
3. The sun's rays must
be hitting the raindrops to create the rainbow.
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