Rainbows – The whole spectrum of colors

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We have all witnessed the creation of the ever-fascinating rainbows, where light refracts at the 0.06 to 2 mm fine water droplets of a rain veil. There, too, the light rays pass from one medium into another, optically denser one. However, without the unique property of this optical principle, which allows the different wavelengths to refract differently, this process would not be possible. We remember back to the prism experiment in physics class: light is sent through a glass prism and fanned out into its spectral colors of red, orange, yellow, green, blue, indigo and violet. Exactly the same thing happens here.

Knowing where to look during a rainstorm lets you predict them. That’s because rainbows actually occur as full circles with a radius of 42° around the antisolar point exactly opposite the sun. They can also appear as double arcs, where the second, outer arc has a radius of 51° and is very difficult to see because of the weak transition. However, we would need to be on a high peak or in an airplane to observe the lower part of the circle, which the earth’s surface normally blocks. Since this is rarely the case, we almost always perceive only the proverbial arc as a segment of the whole circle.

This also explains why we never see a rainbow during midday in summer, when the sun reaches its peak of about 60°. The antisolar point is then so far below the horizon that the arc, with its radius of 42°, completely disappears under the horizon. So the best chance to observe a rainbow is when the sun is flat in the morning or afternoon.

The strength and color of the arcs depend first of all on the quality of the sunlight. Just before sunset, if a significant portion of the short-wave blue-green spectrum has already disappeared, the arc will primarily take on a red hue. And the size of the water drops also plays a role, because the larger they are, the brighter the colors turn out. And the color saturation increases again against the background of a dark sky or a cloud.

In photography, you should be aware that the inside of the arc is typically one stop brighter than its outside. To accurately replicate the five to seven colors, exposure bracketing in 1/3 steps is recommended, depending on the angle. Since the colors change almost permanently with the water droplets dancing in the wind, it is well worth staying with the motif for a while.

By the way, under the aforementioned conditions, the full moon’s reflected light also forms rainbows. Of course, they are much weaker than during the day and, due to our scotopic vision, nearly colorless, but they are still a remarkable phenomenon. If you are lucky enough to be in Yosemite National Park in California in the spring, for example, you can observe this phenomenon on a clear full moon night in the spray of the many waterfalls.

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