The Sound of Science - "Rainbows"

Jul 5, 2019

Sam: I’m Sam Watt from NIU STEM Outreach and this is the Sound of Science on WNIJ. I have a question from Madison today and Jeremy Benson is going to help me answer. Jeremy, Madison wants to understand more about rainbows. Why do we see them and why do they appear in the sky just after a rainstorm?

Jeremy: Let’s start with the basics. Sam, have you ever looked at a straw in a glass of water? What do you notice?

Sam: Well, the straw looks bent at the top of the water.

Jeremy Benson and Sam Watt of NIU STEM Outreach

Jeremy: Exactly! Light bending as it passes from air to water is part of what is causes a rainbow. We associate rainbows with the water in rain showers, but we also need sunlight. Imagine a prism; what do you see if sunlight passes through a prism?  

Sam: All the colors of a rainbow, but not in a bow or arch.  

Jeremy: You’re right. Sunlight is made of all the colors of the rainbow traveling together at the same speed, but when they move through one material into another, such as through water or glass and into the air, they change speed and each color bends a bit differently, spreading out so we can see the different colors. 

To make a rainbow the light must also reflect off the back of the drop. Have you noticed that when facing a rainbow the sun is at your back?  You see a rainbow only when sunlight from behind you travels into a round water droplet where the colors spread out, then reflect off the far side of the drop and exit the rain drop at the correct angle to reach your eye.

Sam: Does that mean I can see a rainbow from one raindrop? 

Jeremy: No, you only see one color from any one drop as the other colors from that drop miss your eye. In fact, if your friend standing next to you also sees a rainbow, you are seeing the colored light from different drops, so you each see a unique rainbow.

Sam: That is cool! Sunlight is made of all these beautiful colors that raindrops release to us in the form of rainbows. That was a great question Madison. Keep your questions coming to us by emailing stemoutreach@niu.edu. This has been the sound of science on WNIJ.

Jeremy: Where you learn something new every day.