Light is fascinating to study with kids because of its interesting properties. Some light is visible and helps us to see. But some light is not visible to us. We use invisible light for heat, for seeing inside our bodies, for looking deep into space, and for many other things. All light is energy and it comes from the sun or from burning and sometimes from chemical luminescence. Another word for light is electromagnetic radiation.
On the image above find the ultraviolet light. It is just to the right of the visible light spectrum. Normally you can’t see it, but if we store it and it builds then we can see just a hint of what is normally hidden.
This particular experiment is a way to trap the ultraviolet light that is always present in sunlight. Here’s what you’ll need:
- 2 clear plastic cups
- tonic water
- tap water
- black paper or black cloth
First, fill one of the cups with tonic water and the other with tap water all the way up to the brim. Label the cups. Now put them in direct sunlight. This is best done around noon or thereabouts on a bright sunny day.
Now hold the black paper or cloth behind the cups. Look at each cup of water through the sides of the cups. Do you see anything different between the two cups?
What you should see is a blue glow on the surface of the tonic water. The blue tint is the ultraviolet light from the sun. Tonic water contains Quinine, which absorbs ultraviolet light. It is then re-emitted as blue visible light. You should be able to see this with any sunlight, but it’s easier to spot when the sun is brightest and most direct.
If you have a black light you can put your tonic water and regular tap water in front of that as well. The tonic water will give off that blue glow like crazy. What type of light do you suppose a “black light” is emitting? What happens if you place your tonic water in front of a normal incandescent light? A florescent light? Are those lights emitting UV rays?
A bit of sunlight trivia for you
If the sun suddenly went from the sky (switched off), would…
A–the place outside look as though the sun is still there (daytime), but no actual sun would be visible in the sky.
B– Everything black out except the sun itself, which would eventually disappear.
C– Everything immediately black out.
D–Everything black out after a period of time.
Answer: D Everything would look and feel normal for 8.3 minutes, after which the light and warmth would abruptly go out…no fading. Light and heat are EM waves which have a finite speed. There’s no reason for a gradual fade-out. Within about 4 seconds after the 8.3 minute mark, light and heat would be gone. The four seconds accounts for the size of the sun (it takes 4 seconds longer for the light to travel from the far side of the sun).
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