Exploring Fluorescence using a Laser

If you’ve ever wondered how glow in the dark toys stay bright even in the dark, this activity is just for you! When light hits a material, it’s either reflected, transmitted, or absorbed as we discovered with the gummy bear activity earlier. However, certain materials will absorb one wavelength and emit an entirely different wavelength, and when this happens it’s called “fluorescence”.  Let’s do an experiment first, and then we’ll go over why it does what it does.

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Materials

• Laser (green)
• Highlighter (pink or orange)
• Diffraction grating
• White paper

1. Point your laser at the wall, making a bright green dot. (Red lasers won’t work with this experiment.)
2. Look at the dot through the diffraction grating. What do you see? How many different colors are there? (If you don’t have a diffraction grating, then simply shine your laser onto a CD and look at the reflected beams.)

We’ll do more on diffraction another time, but just note that a diffraction grating is made up of a lot of tiny prisms that un-mix light into its different colors. That’s why you see several different dots coming from the laser when you pass it through the diffraction grating. If you look at a candle flame through a diffraction grating, you’ll see a whole rainbow, since the white light from a candle is made up of the rainbow. (Image below is a laser through a diffraction grating.) A laser is one color, monochromatic , so you should expect to see only one color through the diffraction grating.

Now let’s try something else…

1. On your white sheet of paper, color an area with your marker.
2. Hold the paper against the wall. You can tape it into place if that makes it easier.
3. Turn off the lights and point a green laser at the highlighter area you colored in.
4. Look at the dot next to the main dot through the diffraction grating.
5. Do you see more than one color now? Whoa!

This is a fantastic experiment because it gives you totally unexpected results! Where did the colors come from when you shined your laser on the highlighter area? And why weren’t they present when you just used a plain white wall?

It has to do with something called fluorescence. When the green laser hits the orange square, the electrons are excited by the laser and jump up to a higher energy state, and then relax back down. When they relax down, they release photons (light particles) that are made up of several different wavelengths. The diffraction grating makes it possible to see those wavelengths individually as a spectrum.

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