The Rainbow Connection

“I do not feel that the same God who has endowed us with sense, reason and intellect has intended us to forgo their use.” Galileo

"Someday we'll find it,
the rainbow connection.
The lovers, the dreamers and me.”
Sarah McLachlan, The Rainbow Connection

Rainbows have always fascinated people. They have inspired poets, and songwriters. Sailors used rainbows to forecast the weather. And most cultures have stories about rainbows handed down from long ago ancestors. Science won't tell you how to get at that pot of gold, but it will tell you how rainbows form, and about things like secondary rainbows and even moonbows.

Of the multitude of colourful cultural stories, my favorite comes from the tropics. The Polynesians have a legend about rainbows and Ina, the goddess who inhabits the Moon. Long ago, Ina fell in love with a mortal man from the Earth and she brought him to the Moon to live with her. The two were deeply in love and remained so as the years passed. But Ina had a problem: she remained unchanged over the years, but her love was growing old – and there was no place for death on the Moon. With heart wrenching sadness, Ina told the old man that he would have to return to Earth. Ina was stricken with grief but she had no choice. She then created a rainbow to allow her love to return to Earth. The Polynesians say that when it rains, it symbolizes Ina's tears, and the rainbow is there to remember her love by.

Magic and legends aside, there is, of course science behind rainbows, and their various cousins which form other optical phenomena in the skies. Rainbows are visible when we face falling rain with the Sun behind us. When we look at a rainbow, we are seeing sunlight reflected back to us through the raindrops. Just how this happens is more complicated. Hard core romantics and nonsciency people may skip a few paragraphs.

Rainbows are products of both reelection and refraction of sunlight. When sunlight enters a raindrop, the water slows it down and acts as a prism, dispersing it into its spectrum of colours. Each colour travels at a different speed and so is refracted to a different angle.. Most of the light passes on through the raindrop, but some of the light rays strike the back of the raindrop at just the right angle, called the critical angle. For water the critical angle is 48 degrees. To achieve this angle the Sun cannot be higher than 42 degrees in the sky. Any sunlight striking the back of a raindrop at an angle greater than the critical angle gets reflected back to the front of the drop where each colour emerges from the raindrop at a slightly different angle. Red light rays refract at 42 degrees while violet rays which move slowest through the water, are refracted at 40 degrees, the other colours are in between these angles..

Only one colour exits from the raindrop at the exact angle to reach the viewer's eyes, so it takes a myriad of raindrops to form a rainbow. As an example, if green light from a raindrop reaches an observer's eyes, the violet light will pass overhead and the red light will fall to the ground in front of the observer.

Because of its angle of refraction, red light rays reach our eyes from raindrops higher up in the sky and violet rays come from raindrops which are lower in the sky. This is why the colours of a primary rainbow change from red on the top to violet on the bottom. Ironically, in a random survey of paintings and illustrations of rainbows – half of them have the rainbows with the colours reversed!

The reason a rainbow can't be seen if the Sun is greater than 42 degrees above the horizon is because the angle is such that the raindrops needed to form the rainbow have already fallen. That's why in midlatitudes you can't see a rainbow during a midday shower, unless you are near a waterfall, or create a rainbow artificially with lawn sprinklers.

What causes a rainbow to form the arc shape? This happens because the rays always travel toward the observer at angles between 40 and 42 degrees from the path of the sunlight. What this means is that anytime you face between a 40 and 42 degree angle from the direction of the Sun's rays – raindrops will direct the colours of the rainbow at your eyes. Because someone in an airplane can also see downward at an angle of 42 degrees, the rainbow can be seen as a full circle.

On many occasions, you can observe two rainbows. This secondary rainbow is formed when sunlight striking the raindrops at such an angle it is reflected twice. It will form about eight degrees above the primary rainbow. The double reflection causes violet to be reflected from higher up and red from lower drops. This means that the secondary rainbow has its colours reversed, violet at the top and red at the bottom. The secondary rainbow is also fainter than the primary because some light is lost in the double reflection. It will form a larger arc across the sky but the bands of colours will be narrower.

Rainbows have a place in weather lore from midlatitude areas and happily this is one bit of lore that is quite accurate. It goes like this:

“Rainbow in the morning, sailors take warning.
Rainbow at night, sailores delight.”

Now this lore sounds very similar to one about red skies at night and morning, but both were important to sailors and farmers. In our rhyme about the rainbow and weather, the science behind it involves cloud movement. In midlatitudes, weather systems typically move from west to east. What this means is that if you see a rainbow in the morning you would be facing west (with the Sun behind you in the east.) So the rain showers would be moving toward you, while in the evening the opposite is true. A rainbow in the evening would be in the east, indicating that the rain had already passed. The exception to this would be a late afternoon where the Sun peeks through clouds to form a rainbow. In this case, more rain could follow.

A very rare form of rainbow can be seen at night with the Moon taking the place of the Sun. This “moonbow” occurs when the moon is very close to full and, as with the Sun, less than 42 degrees above the horizon. And, as with the Sun, there must be rain falling. Because of the dimmer light source, the moonbow's colours are very pale and often it will appear white.

So, can you reach the end of the rainbow? Well sadly,that's about as likely as finding that pot of gold at the alleged end! If you move, you will actually see a rainbow through a whole different bunch of raindrops, meaning the rainbow “moves” with you. But romantics take heart: this also means that everyone sees a different rainbow!