How are rainbows formed. I know it’s about refraction from water droplets, but shouldn’t you see many tiny rainbows instead of one big one.
In: Physics
For simplicity let’s just talk about a single color: red.
When red light comes out of the droplet it’s always at the same angle from the original ray of light and you can see it only if you are at that exact angle so now you can imagine two lines forming an angle with the vertex in the droplet, origin in the sun and end in your eye.
Now we must understand why all the droplets that can make us see red light are in an arc and not in some other shape.
Let’s say that imaginary angle is now a set square with the vertex on the droplet, one edge that follows the ray of light and another edge that points in your direction.
The ray of light edge can’t move because the ray of light doesn’t change its angle.
But if you rotate the set square on that edge you can find all the single points you could be to keep seeing the red light because they all are at the same angle so if you imagine the solid that comes out of the rotation you would find out it’s a cone and the section of a cone is a circle.
Rainbows are circles, not arcs, it’s just that the ground blocks the view.
If you still can’t get why you see an arc, imagine the cone coming out of your eye. If you were shooting out photons from your eye in a cone with that exact angle, all the points on the surface of that cone that intersect a droplet will end in the sun but that cone is the only part of space where you can aim to shoot photons in the sun. Therefore the rainbow must be formed on a section of that particular cone coming off of your eye and the perceived distance of where you see the rainbow depends on how far the droplet is but it’s pretty much just an illusion.
edit: typo
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