Hidden Rainbow

What colors are beyond the violet end of the rainbow?

The science behind UV light. In the “about the science” section of Activity 1: Color Blockers, we talked about what happens when photons, particles of light, run into atoms and molecules. It would probably be a good idea to read that section if you haven’t yet. This activity builds on the ideas discussed there. In this activity the students will use glow-in-the-dark material to see light past the violet of the normal visible rainbow.

Photons of different colors have different energies and blue has higher energy than red. Anything past the violet we can see is going to have even higher energy. These are called “Ultraviolet” or “UV” photons.

In many cases our eyes can’t see that far into the UV. But glow in the dark material can detect it. The glow in the dark material is made up of special molecules called phosphors. Normally to make this stuff glow, it needs to be charged up with white light. When photons from a light source hit the molecules they excite the electrons and make them jump up to a higher energy level. Once they are up there they don’t stay there forever. They slowly fall back down to a lower energy level and something has to happen to the energy they are losing because we know that energy can’t be created or destroyed. The energy the electrons lose pops out as photons and the material seems to glow. But this isn’t all the material can do.

To get an electron to jump from one energy level to a higher level it must be hit by a photon with the right energy. The energy levels. If the difference in energy from one level to the next is the energy a blue photon carries, if the electron is hit by a red photon, it won’t jump up. It will just sit right where it is and the red photon will simply continue on its way. However, if that same electron is hit by a blue or even violet photon, it will jump up and then eventually fall down and emit a photon again. One really cool thing to realize is that this electron, which needs a blue photon to jump up, could be hit by hundreds, millions, quadrillions of red photons and it still won’t budge. It must have the energy of a blue photon. This is true even for the wavelengths of light our eyes can’t see.

When UV light hits the glow in the dark material, it “charges up” the material and causes it to glow. Because there is so much energy in UV photons, they very quickly charge up the molecules in the material and it glows very brightly. So when a rainbow made up of both a regular white LED and a UV LED is shone on a glow in the dark square, the section with UV begins to glow brightly. So even though our eyes can’t see that part of the UV spectrum, the glow in the dark material can tell us where it is.

• Real world connections:
  • Based on what you learned, why do we wear sunscreen outside? Think about what type of light comes from the sun. What kind of light might sunscreen help block?
• Suggestions for drawing, illustrating, presenting content in creative ways:
  • Have students create a drawing with “invisible markers.” A UV light can then be used to reveal what they drew.
• Engineering and design challenges connected to the content:
  • Have students try to design a “UV” shelter with pipe cleaners and various colored construction paper, tin foil, tape, etc. Students can design an “animal” or other object out of pipe cleaners that have UV beads attached to it. The goal is to leave the animal in the shelter, expose it to UV light, and have the beads remain white.