Curve of Rainbow
Name: Lee R. W.
A friend of mine was visiting his young granddaughter and
they viewed a rainbow together. Her question to him was not about the
colors but was fascinating to me for such a young observer. She asked
"Why is a rainbow curved and why is it in a continuous curve and not in
small sections?" The grandfather seeks help and I would love to see this
myself. The idea that light is broken down in passing through raindrops
which are basically spherical may be part of this but somehow it seems
Lee R. W.,
A rainbow is curved due to angle requirements. Light from the sun that
bounces through a drop of water, comes into your eye, and appears red must
bounce at a specific angle. Join two sticks together at a set angle. Place
one end at a dot to represent the sun. Place the other toward your eye.
The place where the sticks join, where the water drop is, can be rotated
around in a circle. A rainbow is shaped like a circle.
You never see the entire rainbow because of the horizon. When you look at a
rainbow, the Earth gets in the way of most of the circle. The portion you
see is just the top portion. Draw a circle and cover a little more than
half of it. What is left is the shape of a rainbow.
Dr. Ken Mellendorf
Illinois Central College
Consider that the raindrops are acting as prisms by separating the
different colors of light. Each color gets reflected at a different
angle. Any particular color is reflected at a single angle between the
source (the sun) and the receiver (you, the observer). The arc
described by the individual colors of a rainbow is defined by this
requirement that the angle from sun to rainbow to observer is fixed.
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Update: June 2012