Why are rainbows always a semi-circle (arc) shape?

Answer 1

This optical phenomenon has a lot to do with the angle of the observer and the way spherical water droplets refract and split the white sunlight into it's constituent colours of the spectrum.

The arc is actually part of a complete circle which we can never actually get to see unless we view it from above in a plane (see the reference) - given by an angle of view of 84°.

Hope this helps.

Answer 2

Because you're seeing a primary rainbow at a constant angle (ca. 42deg) from the sun and a raindrop. If you were high enough up in the air, you would see a complete circle, but you only see the arc in the sky because there are no raindrops on the ground.

The higher the sun in the sky, the less of an arc you see, and vice versa.

Answer 3

I saw a circular rainbow from the top of Mt Hellvellyn in the English Lake District once.

Answer 4

what you see is always an arc of a circle.

If you see a rainbow from a passenger jet, you see the whole circle

Answer 5

That's just how it appears from our limited vision. If you were to fly up in an airplane you would see it as a full circle. It has to do with the bending and refracting of color and light.

Answer 6

Its just down to perspective. We are on the ground so we are limited in the angles we see them.

Answer 7

They are circular it is just we can't see the bottom bit.

Answer 8

Because you couldn't call them "bows" if they weren't semicircles. "Somewhere over the rainparabola" just doesn't sound as good.

Answer 9

rainbows are shaped like monkeys, i don't know what the heck you've been looking at....

Answer 10

They are not, they are circular.