What Causes a Rainbow?
After a shower of rain there is often a rainbow. Rainbows form when the sun begins to shine through the clouds while the air is still filled with raindrops. The raindrops act as tiny prisms to separate sunlight into the colors of the spectrum, and an arch of colors appears in the sky.
The many colors in a rainbow blend into one another so that we rarely see more than four or five clearly. You’ll miss it if you face the sun. A rainbow can only be seen if you face the sky with the sun at your back. Little rainbows can often be seen in the spray of a lawn sprinkler.
Rainbows can be full circles; however, the average observer sees only an arc formed by illuminated droplets above the ground, and centered on a line from the sun to the observer’s eye.
In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.
In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colors reversed, with red on the inner side of the arc.