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
It’s because the sun is very far away.
When a single raindrop refracts the light, it does it in a straight line, red on top, violet on bottom. The angle this line comes out of the droplet depends on the angle between the sun, the drop, and you.
Because the sun is very far away, we can consider it a “point source”. That means all the light is coming from the exact same point in space. The light source has no width or height, just a single dot of light.
Now, since we can consider the sun a point source, think about the shape of a rainbow. It’s a circle, though usually cut off by the horizon. So we have a circle and a point, draw some lines between them and you have a cone!
What’s happening here is that when water droplets refract the light, the angle the light comes out is relative to the angle it comes in. Since the sun is a point source, the angles add up to form a circle! Think about the cone we drew from the sun to the rainbow. Light comes from the point, travels to the base edge at an angle, then hits a water droplet that refracts it at the *opposite* angle. This places the point of observation somewhere *inside* the cone, which is why you can only see rainbows if the sun is behind you!
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