In this experiment, water is our prism. A prism un-mixes light back into its original colors of red, green, and blue. You can make prisms out of glass, plastic, water, oil, or anything else you can think of that allows light to zip through.


What’s a prism? Think  of a beam of light.  It zooms fast on a straight path, until it hits something (like a water drop).  As the light goes through the water drop, it changes speed (refraction). The speed change depends on the angle that the light hits the water, and what the drop is made of.  (If it was a drop of mineral oil, the light would slow down a bit more.) Okay, so when white light passes through a prism (or water drop), changes speed, and turns colors.  So why do we see a rainbow, not just one color coming out the other side?


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The secret is because the light is made up of different wavelengths, and each gets bent by different amounts when they hit a new material. So one wave changes speed to red, another to yellow, another to green, etc. when the beam hits the prism. And water drops are tiny prisms.


The light passing through a water drop gets refracted twice, not once.  The first time is when it enters the water drop, the second when it bounces off the other side of the drop and reflects back through the water drop and out again (some of the light does make it out the other side of the drop, but most of it bounces back).  When the light emerges from the water drop, it changes speed again, and presto! You have a rainbow.


Natural rainbows (the ones that you see after it rains) happen when water drops (tiny prisms) in the air are hit by sunlight from behind you at just the right angle (which is relatively a low angle, near the ground).  The best rainbows can be seen when half of the sky is darkened with rainclouds and you’re in a clear patch with sun behind you. And guess what?  You can even see a nighttime rainbow (called a moonbow), although they’re pretty rare, usually near full moon.


Here’s what you do:


Materials:


  • mirror
  • shallow baking dish
  • water
  • sunlight

 
Download Student Worksheet & Exercises


Set a clear tray of water in sunlight. Lean a mirror against an inside edge and adjust so that a rainbow appears on the wall. You can also use a light bulb shining through a slit in a flat cardboard piece as a light source.


Troubleshooting: This is one of the easiest experiments to do, and the most beautiful. The trouble is, you don’t know where the water shadow will show up, so make sure you point the mirror to the sky and play with the angle of the mirror until you find the wavering rainbow. Because the shadow is constantly moving, you can snap a few pictures when you’ve got it so you can look over the finer details later. If this project still eludes you, take a large sheet and use it instead of the tiny index card.


Exercises


  1.   What serves as the prism in this experiment?
  2.   What property can help make something a good prism material?
  3. What are some other items that could be used as prisms?

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