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Student Guide

Color Blockers

Experimenting with color absorption and transmission

What colors of light are absorbed and transmitted by different colors of water?

This resource was originally published in PhysicsQuest 2014: Quantum.

What colors of light are absorbed and transmitted by different colors of water?

  • Electromagnetic spectrum inhibitor tablets
  • Diffraction grating viewer
  • Red, green, and blue LEDs
  • Batteries
  • Flashlight
  • Water
  • Three clear plastic cups
  • Tape

As the ziploc bag commercial says: “Yellow plus blue makes green.” When you mix different food coloring or paints together, you will get new colors. But why, exactly? Why do we see the colors we do through transparent things such as ziplock bags and water? This experiment will use rainbows to explore why.

Before the experiment
  • What are the colors of a rainbow?

  • What color do you think is the most “energetic?” Why?

Setting up
  • Fill the three plastic cups halfway full of water.

  • Color the water red, yellow and blue using thee “electromagnetic spectrum inhibitor tablets.”

  • Tape the flashlight to the edge of a table or chair so that it is pointing straight up.

  • Place the battery between the red LED wires, making sure to put the longer leg on the positive side of the battery (the one with the “+” sign).

  • Tape the lit LED to the edge of the table or chair about a foot away from the flashlight.

During the experiment
Collecting data
  • Look through the diffraction grating viewer at the flashlight. What do you see? Draw it.

  • Tape the diffraction grating viewer over the cup with red water and hold it over the flashlight.

  • What do you see? Draw it.

  • Do the same thing with the yellow and blue water, drawing what you see.

  • When the white light passes through water, some colors of the light are absorbed, which means they are blocked; and some are transmitted, meaning they pass through. The colors that you see are the ones that are transmitted. The other colors are absorbed. Fill in the table.

  • This time we’re going to do the same thing, but instead of looking at white light, we’re going to look at specific colors of light, red, blue, and green.

  • Predictions: What colors do you think you would see if you looked through the diffraction grating viewer at the red LED?

  • Try it and record your results in the table.

  • What do you think would happen if you looked at the LEDs through two colors of water at once?

  • Try it and record your results in the table.

Analyzing data
  • Were your predictions correct? What did you find surprising?

  • What other combinations do you think would be interesting?

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?

    1. Yellow and blue
    2. Everything but blue and yellow
    3. Everything but green
    4. Everything but blue
  • What does it mean when light is ‘transmitted?” What about “absorbed?”

  • Why do oceans look blue?

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