The colors of the rainbow are red, orange, yellow, green, blue, and purple but is that all? Is there anything just beyond what our eye can see? And if so, is there any way we can detect it? In this activity you’ll see a little piece of what’s hidden beyond the rainbow we know.
Student Guide
Hidden Rainbow
Experimenting with colors we can’t see
What colors are beyond the violet end of the rainbow?
This resource was originally published in PhysicsQuest 2014: Quantum.
What colors are beyond the violet end of the rainbow?
- Glow in the dark vinyl
- Binder clips
- UV LED (the small one)
- Two batteries
- CD
- Flashlight
- Adhesive tape
Intro
Before the experiment
What are the colors of a rainbow? Draw them.
Do you think there is light beyond what we see? Where would it fit on the rainbow?
How do you think it might be possible to detect colors of light you can’t see?
Setting up
Attach binder clips to the CD so that it can stand up.
Attach a binder clip to the flashlight so that you can put it on a table and aim it at the CD. It may help to take the key ring off.
Put the two batteries together so that the smooth side of one is against the bumpy side of the other. Don’t tape them together yet.
Take the UV LED and straddle the batteries with the legs of the LED making sure the longer leg is on the smooth side of the battery. The LED should glow purple.
Tape the whole thing together so that it stays lit.
Put it in a binder clip just like the flashlight.
During the experiment
To be able to “see” the hidden part of the rainbow, the first step is to make a rainbow.
In a dark room, set the CD at an angle about 1-2 feet away from a wall.
Put the flashlight at an angle about 6 inches from the CD so the light from the flashlight bounces off the CD onto the wall.
Move it around until you can see a rainbow on the wall. This might be a little tricky and will require some work, but keep trying, you can do it.
What colors do you see? Draw it.
Now put the glow in the dark square on the wall so the rainbow is projected on it. What do you see now?
Take the glow in the dark square away so the rainbow is just on the wall again.
Bring the UV LED up right next to the flashlight so that they are both being projected on the wall.
What colors do you see now? Is anything different?
Again put the glow in the dark square on the wall so that the rainbow is projected on it.
What do you see right away and what do you see if you wait a few seconds?
Take the glow in the dark square out of the way of the rainbow. What does it look like?
The glow in the dark square glowed:
- More with the UV diode, which has more energy than other colors of light.
- The same with or without the UV diode.
- More with the UV diode, which has less energy than the other colors of light.
- Less with the UV diode.
Conclusion
What would happen if you looked through all three colors of water at the white light?
Which colors are absorbed by blue and yellow water?
- Yellow and blue.
- Everything but blue and yellow.
- Everything but green.
- Everything but blue.