Why are rainbows "circular"...???

Asked by Somesh Meena | 27th Jan, 2014, 04:05: PM

Expert Answer:

When sunlight and raindrops combine to make a rainbow, they can make a whole circle of light in the sky. But it’s a very rare sight. Sky conditions have to be just right for this, and even if they are, the bottom part of a full-circle rainbow is usually blocked by your horizon. And so we see rainbows not as circles but as arcs in our sky.

When you see a rainbow, notice the height of the sun. It helps determine how much of an arc you’ll see. The lower the sun, the higher the top of the rainbow. But if you could get up high enough, you’d see that some rainbows continue below the horizon seen from closer to sea-level. Mountain climbers sometimes see more of a full-circle rainbow, though even a high mountain isn’t high enough to show you the whole circle.

Pilots do sometimes report seeing genuine full-circle rainbows.

Rainbows are caused by rays of sunlight that reflect back toward the sun after hitting spherical water droplets, such as those found in a raincloud or in rain itself. The light does not reflect directly back toward the sun, but rather are offset at approximately 42 degrees, the "Rainbow Angle". Thus, you will see the rainbow in a perfectly circular arc, whose radius is 42 degrees and whose center is directly opposite the sun. Since blue light travels at a slightly different speed within the water droplet then red light, the angle is just a little bit different for different colors, leading to the lovely color bands in a rainbow.
This is also why there is no "end of the rainbow": circles have no ends.

Answered by Komal Parmar | 27th Jan, 2014, 05:21: PM

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