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Credit: E. Wiebe

A sudden brief rain shower provided perfect conditions for a beautiful rainbow over the southern Gulf Islands in British Columbia. Rainbows can be seen when you face falling rain drops with the sun behind you. They are caused by a combination of optical phenomena called reflection, refraction and diffraction. Each droplet causes a ray of sunlight to travel toward you by internal reflection. This ray is refracted into it's composite colours by the difference in refractive index in the droplet and the air and joins with rays from all of the other drops that happen to travelling toward your eyes. These rays combine via diffraction (especially interference) to form the image of the arch. Normally you see only part of the circular rainbow. It is possible, if the sun is low, near the horizon or if you happen to be very high up to see the whole circle.

In this case a secondary rainbow is visible as well with the colour order reversed. This is formed from rays that reflected twice in the droplets before travelling back toward your eyes. A number of other interesting effects can occur including bows that are caused by rays reflecting from a surface first. This is most common over water. Additional, supernumerary*, bows are sometimes seen inside the bright primary arch. These rainbows are caused strictly by interference of rays travelling different lengths within the drops, perhaps in showers with many different droplet sizes. This is analagous to the sheen you observe when a thin layer of oil is placed on a water surface.

Finally, notice the distant cumulous clouds. These are forming over the mainland south of Vancouver. They are forming because of local heating and convergence of flow over land.

* Why use a short word like "extra" when you can use "supernumerary"?


UVic / SEOS / Climate Group / About Front Page Picture / Gulf Islands Rainbow Last updated: Tuesday, 04-May-2010 11:25:07 PDT
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