Rainbows

[Illustration: Water droplet refracting spectrum]

In ancient Hawai‘i a rainbow, or ānuenue, symbolized the presence of a god or a chief.  Today the Hawaiian Islands’ tropical climate and misty rainshowers provide many opportunities to see a rainbow painting the sky with colors. 

An ancient Hawaiian proverb states, “Kau ka ‘ōnohi ali‘i i luna.” (“The royal eyes rest above.”), referring to the sight of “a rainbow—a sign that the gods are watching the chiefs.”[i]

Light Dispersion

Rainbows are caused by drops of water in the atmosphere, and are spectacular illustrations of a phenomenon known as dispersion, in which light is separated into different colors arranged according to their frequencies. 

Different frequencies of light travel at different speeds when passing through transparent materials, such as water droplets.  Water droplets cause the various components of light to refract differently, bending the rays at different angles.

For example, if violet light from a single water droplet enters your eye, the red light from the same droplet will fall below your eye, and so you will not see it.  To see red light you have to look at a raindrop higher in the sky (e.g., higher on the rainbow).  Since our eyes see different colors from droplets at different heights in the sky, when there are many water droplets in the sky we will see a rainbow. 

[Illustration: Diagram—Anatomy of a Rainbow]

The Anatomy of a Rainbow

Each water droplet in a rainbow disperses a full spectrum of colors, but from where you’re standing you will see only one of the colors from any particular drop.  You will see the color that refracts at just the right angle to reach the place where you are standing. 

For example, you’ll see red when the angle between a beam of sunlight and the dispersed light (at the water droplet) is 42 degrees, and violet when the angle is 40 degrees. 

The top of a rainbow is red and the innermost arc is violet—this is because a water droplet bends violet the most and red the least.  The light forms a “bow” because when you see a rainbow, you are actually seeing a three-dimensional cone with the apex of the cone at your eye. 

To help understand the geometry of this, put your arm out at a 42-degree angle to the side and then move it to 45-degrees up from horizontal in front of you, and then to the other side—you have traced out the shape of a bow.

Double Rainbows

Not uncommon in the Hawaiian Islands is the beautiful sight of a double rainbow.  A double rainbow is actually two completely different rainbows, one directly over the other. 

The secondary rainbow is larger than the primary rainbow, and has its colors reversed, with violet on the outside (the top), and red on the inside (the bottom).  Double rainbows are caused by a double reflection within the raindrops. 

Ka wela o ka ua.

Heated rain.

Warrior chiefs in feather cape and helmets. 

They look like little rainbows—rain “heated” by the sun.

                                                            (Pukui: 1664-180)

Seeing Rainbows

To see a rainbow, you have to have your back to the sun because rainbows always occur opposite the sun.  Generally, the sun also has to be less than 42 degrees above the horizon and this only occurs in the morning and evening (the most common times to see rainbows).

[Photograph: Rainbow]