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Name: Lee R. W.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 11/15/2004


Question:
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 very incomplete.


Replies:
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
Physics Instructor
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.

Greg Bradburn



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