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Wednesday, October 27, 2010

Now You See Me, Now You Don't!

Another theme that appears in movies is invisibility. In 1933, The Invisible Man told the story of a mad scientist who discovered a secret formula for invisibility, which also made him crazy. The comedy film The Invisible Woman was released a few years later (music is not from the original movie). In this film, it was a machine, not a formula, that made the person invisible. The amazing thing to remember about these movies is that they were made without any computer-generated special effects. In more recent films, Harry Potter's invisibility cloak and the Invisible Woman in Fantastic Four, use computers to make people seem to disappear.

But things don't really disappear in real life, do they? Well, actually, sometimes they can. Take a careful look at the photo at right. The beaker on the left contains vegetable oil; the one on the right has water. You can clearly see the glass stirring rod through the water, but it is nearly invisible in the oil. You can try this at home.
Why is the glass rod invisible in the oil? To explain this, we need to start with the speed of light. In a vacuum, light travels at an incredibly fast speed of 671 million miles per hour. If you could move as fast as light, you could get to the sun in about 8 minutes! When light passes through something, whether air, water, or glass, it slows down (not by much, but by a measureable amount). In air, it's "only" going 670 million mph. It's speed is 500 million mph in water, and 450 million mph through glass. Yup, light becomes a real slowpoke when moving through something other than a vacuum!
One of the reasons that you can see glass, even though it's transparent, is that the light rays bend when they pass through it. When you look at the glass rod in water (on the right), the light is bent by both the glass and the water, and your eyes and brain can see it. However, oil and glass have just about the same index of refraction, so the light is only bent once. You can see the oil, but it's very hard to spot the glass rod in the beaker. So it's invisible!
(Thanks to the Chemistry Club at Fairleigh Dickinson University in Madison NJ! Dr. B learned about this experiment at an event where they did this demonstration.)

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