Phosphorescence: Writing with Light
[Please be sure the flashlights stay in the room- we actually lost most of them last year]
[Shopping List: Glow board (a foam board painted with glow-in-the-dark or phosphorescent paint); several LED keychain flashlights of different colors, particularly red, green, blue, white and ultra-violet; large UV flashlight; some fluorescent balloons, posters or other materials and a blacklight]
- Hold one of the LED keychain flashlights fairly close to the glow board and turn it on. Move the flashlight back and forth. Depending on the color of the flashlight, you may see a bright glow from the paint, a dim glow, or nothing at all. Make sure the kids don't press or touch the flashlights to the board as it will scratch and damage it.
- Try all the different colors. Which ones produce a glow, and which don't?
- Hold your hand close to the board, then using a white or UV flashlight (particularly the big one), turn it on for a second or two, turn it off, then move your hand away and look at the board. You should see the shadow of your hand "frozen" on the board for a few seconds.
What's Happening: The board has been painted with glow-in-the-dark paint, which contains a phosphorescent pigment. When light with sufficient energy shines on a phosphorescent material electrons in the atoms absorb this energy and are excited or raised to a higher energy state. This excited state is very unstable however, and the electrons will soon fall back into their normal or ground energy state, and when this happens a photon of light is emitted, producing the glow you see. This is also what happens with fluorescent materials, the only difference being how long it takes the excited electrons to fall back into their ground energy state. For fluorescence, this happens immediately, which is why the bright "neon" glow disappears as soon as you stop shining your flashlight, while for phosphorescence this may take several seconds or even minutes for all of the electrons to return to their ground state, producing a long lasting glow.
The color of the light is very important. Light is just a form of electromagnetic radiation or waves, which includes all the colors we can "see" (called visible light), as well as radiation we can't see, like infrared, ultra-violet, radio, microwave, x-rays and gamma rays. These waves can all be characterized by their wavelength and frequency, which also determines the amount of energy they contain. Waves with long wavelengths and low frequencies (like radio or infrared) have less energy than those with short wavelengths and high frequencies (x-rays and gamma rays.) The rainbow of colors we see in visible also indicates how much energy the light has. Red has the longest wavelength and thus lowest energy, green is in the middle, and blue or violet has the shortest wavelength and highest energy. We can't really see ultra-violet or UV waves, but UV or blacklight bulbs actually emit much of their light in the violet part of the spectrum which we can see. What we call "white" light is just light which contains all of the visible colors.
A phosphorescent or fluorescent electron can only be excited by electromagnetic waves above a certain threshhold energy. For the paint on our board this means only light with a short wavelength will make it glow. Blue or shorter wavelengths of light have enough energy, and the shorter the wavelength the brighter the glow will be (since more electrons will be excited), thus UV works the best. Green light has just barely enough energy to make a little glow (if the room is very dark you might see it), but any light with longer wavelengths- like red- doesn't have enough energy to produce any glow at all. Of course the white flashlight contains all colors, so there is some blue and violet present capable of exciting the paint.
You will also notice that the board always glows with a greenish yellow color, regardless of the flashlight color used to excite the electrons. This is because the photons or light emitted when the electrons fall back to their ground energy state are always the same frequency or color, which is determined by the material, not the light which excited it. For our paint this color is greenish yellow. The photons emitted also have less energy than those which excited them in the first place because some of the energy is lost in the process, thus the color of the glow has a longer wavelength than that of the flashlight which produced it.
Variations: Adult volunteers may demonstrate our UV laser pointer, which can produce a glow on the board from several feet away since the energy in the beam does not spread out and diminish like the flashlights. While the red light cannot produce a glow from the board, it can still do something very cool. Shine a red flashlight through your finger (small kids may even try this with their hand.) Body tissues including bone are almost completely transparent to longer wavlengths like red, i.e. the light passes through your finger without being absorbed as shorter wavelengths like green and blue are. Try it with one of those flashlights. It is also interesting to note that while even your bones are invisible with a red light, your veins will appear as dark lines. This is because the blood in your veins is actually much darker red than the bright red blood in your arteries, and thus absorbs red light.
|
