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Question
What are first and second order rainbows?
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Solution
Rainbows result from refraction of sunlight in falling water droplets plus reflection of the light from the back of the droplet.
If during a rain shower you can see the shadow of your own head, then you are in a position to see a rainbow if conditions are favorable. The rainbow forms a circular arc around the anti-solar point, which is located in the shadow of your head. You can search for the shadow of your head to find a rainbow in a waterfall, or even in the spray from a hose or sprinkler.
Primary Rainbow
The primary rainbow forms between about 40∘and42∘ from the antisolar point. The light path involves refraction and a single reflection inside the water droplet. If the drops are large, 1 millimeter or more in diameter, red, green, and violet are bright but there is little blue. Such large droplets are suggested by the rainbow at right. As the droplets get smaller, red weakens. In the fine mist, all colors except violet may disappear. Even finer fog droplets, smaller than 0.05 mm, produce the white rainbow or fog bow. The secondary rainbow involves two reflections inside the falling droplets.
Rainbows are not seen in midday since the whole 42° circle is below the horizon at most latitudes. So rainbows tend to be seen most in the later afternoon when a thundershower has passed and the sun from the west is illuminating the receding edge of an eastwardly moving raincloud. It is possible to see the entire circle of the rainbow from an airplane since there can be falling droplets both above and below you.
Secondary Rainbow
The secondary rainbow is about 10° further out from the antisolar point than the primary bow, is about twice as wide, and has its colors reversed. According to Schaaf, the light of the secondary bow is one-tenth the intensity of that of the primary bow, given the same viewing conditions.
If during a rain shower you can see the shadow of your own head, then you are in a position to see a rainbow if conditions are favorable. The rainbow forms a circular arc around the anti-solar point, which is located in the shadow of your head. You can search for the shadow of your head to find a rainbow in a waterfall, or even in the spray from a hose or sprinkler.
Primary Rainbow
The primary rainbow forms between about 40∘and42∘ from the antisolar point. The light path involves refraction and a single reflection inside the water droplet. If the drops are large, 1 millimeter or more in diameter, red, green, and violet are bright but there is little blue. Such large droplets are suggested by the rainbow at right. As the droplets get smaller, red weakens. In the fine mist, all colors except violet may disappear. Even finer fog droplets, smaller than 0.05 mm, produce the white rainbow or fog bow. The secondary rainbow involves two reflections inside the falling droplets.
Rainbows are not seen in midday since the whole 42° circle is below the horizon at most latitudes. So rainbows tend to be seen most in the later afternoon when a thundershower has passed and the sun from the west is illuminating the receding edge of an eastwardly moving raincloud. It is possible to see the entire circle of the rainbow from an airplane since there can be falling droplets both above and below you.
Secondary Rainbow
The secondary rainbow is about 10° further out from the antisolar point than the primary bow, is about twice as wide, and has its colors reversed. According to Schaaf, the light of the secondary bow is one-tenth the intensity of that of the primary bow, given the same viewing conditions.
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