Tuesday, May 24, 2011

Experiments with Light

Black Light & Glow Sticks

Ivy and a couple of friends took an afternoon to play with black light.  We discussed what black light is and where it falls on the light spectrum.  They made notes on which things "glowed" in the light and which were unimpressive.  We also had some fun figuring out how glow sticks work and noted that they glow in the black light even when they're not activated!

First, let’s talk about the light. The reason black lights are called "black lights" is because they give off very little light that our eyes can see. Visible light contains a spectrum of colors ranging from red, through orange, yellow, green, and blue, to violet or purple. Beyond violet light in the spectrum is ultraviolet light, which our eyes cannot detect. 

You may have heard of ultraviolet light if you know about sunburn. Sunburn is caused by a type of ultraviolet light, which scientists call “ultraviolet B” (UV-B). UV-B is higher in energy than the light from black lights, which is called “ultraviolet A” (UV-A). Black lights will not give you a sunburn.

If we can't see ultraviolet light, why does the petroleum jelly glow under the black light?
Most of the time when we look at an object, we see light reflected from the surface of the object. But with a black light, there isn't much visible light, so simple reflection of light doesn't account for how bright the jelly glows. Petroleum jelly contains substances called phosphors. A phosphor absorbs radiation and emits it as visible light. So the phosphors in the jelly are absorbing the invisible ultraviolet radiation from the black light and emitting visible light.

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How do glowsticks work?

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