Rainbows are one of the most famous and beautiful phenomena in our Hawaiian Islands, and anybody who grew up here has seen the brilliant bands of light over the Ko'olaus or above Diamond Head. But what is a rainbow? Simply put, we've learned that a rainbow is a product of light refraction, as the white light from the sun behind the viewer passes through the raindrops and is diverted to the viewers eyes. The reason the rainbow makes a "bow" shape is because the Earth blocks a good deal of the sun's light, resulting in the semicircular arc form the light that does pass the Earth.

Visual aids helped me to understand this concept more fully. The following section is taken from the University of Carolina's website:

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The traditional diagram to illustrate this is shown here as adapted from Humphreys, Physics of the Air. It represents the path of one light ray incident on a water droplet from the direction SA. As the light beam enters the surface of the drop at A, it is bent (refracted) a little and strikes the inside wall of the drop at B, where it is reflected back to C. As it emerges from the drop it is refracted (bent) again into the direction CE. The angle D represents a measure of the deviation of the emergent ray from its original direction. Descartes calculated this deviation for a ray of red light to be about 180 - 42 or 138 degrees.

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It was this understanding of the physical aspects of rainbows that helped me to explain the following picture, taken from Kailua Beach near sunset:

It looks like a little patch of rainbow floating out over Flat Island. I though of two possible explanations for this phenomenon, and in reality it could be both. The first is that there are only water droplets in the air (rain) at that one spot. The second is that the setting sun is being blocked by some landscape feature, most likely the Ko'olau mountain range, and only that relatively small section of sky is receiving direct sunlight. In both cases, the sun is setting behind the camera, which meshes with the UCar diagram above. This is a cool picture made evn cooler by an understanding of physics!!1! :D