What Causes Rainbows?
When the sun is out during or right after a shower, a rainbow appears in the sky. When sunlight strikes the drops of water in the air, the raindrops act as tiny prisms. The raindrops separate the light into bands of red, orange, yellow, green, blue, and violet, and reflect the colors back to our eyes in the form of a rainbow.
To see the bow, you must be standing with the sun behind you and the raindrop filled sky in front of you, for a rainbow forms in the sky opposite the sun. Little rainbows can often be seen in the spray of a lawn sprinkler.
The technical details of rainbow formation were first analyzed by Isaac Newton in 1665. His brilliant optics work concerning reflection and refraction certainly does not detract from the beauty and promise of the rainbow. On the contrary, Newton’s scientific insights show the marvelous complexity of creation.
The rainbow light is reflected to the eye at an angle of 42 degrees to the original ray of sunlight. The bow shape is actually part of a cone of light that is cut off by the horizon. If you travel toward the end of a rainbow, it will move ahead of you, maintaining its shape.
Thus, there is no real end to a rainbow. Because the 42 degree angle is measured from each individual observer’s eye, no two people see exactly the same rainbow. Every person is at the center of his or her own particular cone of colored light. From the high vantage point of a mountaintop or an airplane a complete circle of rainbow light sometimes can be seen.
The bright, primary rainbow has red on the outer edge and blue within. Higher in the sky there is always another, dimmer rainbow with the order of colors reversed. This secondary rainbow results from additional reflection of sunlight through the raindrops. It is most visible when there are dark clouds behind it.
Look for the second bow high in the sky the next time rainbow colors appear. Some observers have even reported seeing third and fourth rainbows above the first two.