At the End of the Rainbow

Explanation

Rainbows become visible when it rains and the sun shines at the same time. The array is important, however. The sun has to be behind you while you watch in the direction of the rain clouds. The raindrops reflect the light of the sun back into your direction. Parts of the reflected light reach your eyes: A rainbow is visible.

Rainbows always form in the shape of arches. The reflected sunlight leaves the raindrops at a specific angle. The raindrops reflect the light coming from the sun at angle of about 42° into the eye of the observer. For instance, this angle can be found between the direction of the incoming sunlight and the line from the highest point of the rainbow into our eyes. If we look to the left, this angle can be found in the left part of the rainbow. However, if you look at a point that is on the horizontal side of the highest point of the rainbow, the angle would be bigger than 42° so that the raindrops do not reflect any light from these parts. Light is only reflected by the drops in the circle so that an arch is formed.

The circular arc of a rainbow can be explained with a set square, also called a triangular ruler. The center of the circle is on the extension of the line from the sun through your head. Therefore the shadow of your head points to the center of the rainbow. First you need to align one of the short sides of the set to this line, so that one tip of it points to your eye and the other one points to the center of the rainbow. If you target the rainbow in this position, you will see it in the extension of the other short side. Because the colourful light always leaves the arch at a specific angle, the top of the square set always points at the rainbow. If you rotate the square set, you can trace back the circular shape of the rainbow.

An observer next to you will see a different rainbow. To them, the rainbow forms at an angle of 42° to the line between the sun and the shadow of their head. Because your lines run next to each other, your observed rainbows are slightly shifted as well.

So if you try to reach the end of the rainbow and you move towards it, the rainbow moves as well. No matter how far you will walk, you will never reach the end of the rainbow.

Now you can either go back to the task page or you can go on if you want to know more about the colours of a rainbow.