We all know that light follows Rectilinear Propagation and it sometimes bend while diffraction, but i never understand the concept of rainbow and the reason why it is a curve. I have read books and all and everywhere it says that white light passes through raindrops and boom! It is now a circular arc of 7 lights. I understand the phenomenon of refraction and know light of different wavelengths separate when passes through a prism. But after passing through prism, those light travels straight.
But why is rainbow curve? 🌈
In: 21
First of all, light doesn’t always travel in a straight line, it can be *bent*. For example, put a straw in a cup of water, then look at it from the side. Notice how the straw in the water doesn’t seem to connect with the part in the air? That’s because light is being bent as it hits the water.
A good analogy would be in imagine a rolling log on a street. Then part of the log hits a patch of grass and the log starts to spin, because the log can roll faster in the street than in the grass. The path of the log is being bent just light light can be bent when it hits a different material.
In the case of a rainbow, imagine a prism sending out light of different colors. Put a piece of paper in the “blue” light, that’s your “eye” seeing the blue part of a rainbow in the sky. Now move the prism around, depending on how you move it the paper might see “red” or “green” or nothing at all if you’ve moved the prism so that it doesn’t cast it’s colors on the paper any more.
If you did the experiment properly, you’d discover that there is a certain amount of left-right movement you can do so that each of the colors falls on the paper, beyond that movement the light misses the paper and you can’t see it. Additionally, there is some some up/down motion you can do so that the colors still fall on the paper.
What I’m trying to explain is if you made a “map” of all the left/right and up/down places you could put the prism to still have the colors it casts hit the paper, you’d get an arch.
When you have water droplets suspended in the air they act as tiny little prisms, spreading out their colors. Instead of moving a single prism, all the water droplets act as a series of tiny prisms, each casting their colors at the same time. Just like in our experiment (only with your eyes instead of paper) the rainbow you see is the combination of all the lights and colors from all the prisms at the same time. And guess what, it looks like an arch.
In short, you see a rainbow as an arch because that’s the water droplets in the arch form the “series of prisms” that your eye can see from where you’re standing.
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