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3D Chalk

On my recent trip to Memphis, my Mom, my sister, my nieces, and I played with some of the new 3D Sidewalk Chalk from Crayola. Now, I usually don't promote products, but this one is fun, and has lots of science in it. If you don't want to buy the product, some of the options have the glasses packaged so you can look through them in the store, but I warn you, if you play with them you will wind up buying them.

The idea is that you put on the glasses, and then draw on the sidewalk with the chalk. Depending on the color of the chalk, it may seem to float over the sidewalk, or seem to be sunk down into it.

Now, if you have ever seen a 3D movie, you have probably taken off the 3D glasses at some point, and seen what a mess the screen looks like. That is because the screen actually has two pictures on top of each other. One image is for your right eye to see, and the other is for your left eye to see. If both images were the same, it would look flat, just like a normal movie. For a 3D movie, the images are offset a bit, but some parts of the image are shifted more than others. The further apart the two images are; the closer the object will seem to be. For example, if a ball is supposed to look as if it is coming out into the audience, without the glasses, you would see two separated images of the ball.

On the other hand, if the two images are on top of each other, the object looks very far away. Without the glasses, the distant background still looks fairly clear.

With a 3D movie, the lens of your right eye blocks the left eye image, so your right eye sees only the right image. It can do that either with color (the glasses that have one red lens and one blue lens) or with polarized light (the glasses look like sunglasses.) The lens for your left eye blocks the right eye image, so your left eye sees a different image. When your brain puts the two images together, you see things in 3D.

Now, lets get back to the chalk. In this case, you have only one image, not two like a 3D movie. So how do the glasses let you see in 3D? To find out, you need to look thorough a pair of the glasses. Close your right eye and look through the glasses at something white. A white sign works very well. You should notice that one side of the sign has a blue stripe and the other side has a red stripe. That is because the lens (called a diffraction grating) is shifting the colors, but it does not shift them evenly. It is shifting the blue end of the spectrum to one side, and the red end of the spectrum to the other side.

Next close your left eye, and open your right eye. Look at the sign again, and you will find that the stripes have changed places. That is very important, because it means that your right eye and your left eye are seeing the colors in different places. The red end of the spectrum (reds and oranges) is shifted further apart, so things that are red will look as if they are closer, floating above the pavement. The blue end of the spectrum (blues and purples) is shifted in the opposite direction, so objects that are blue or purple seem to be at a level below the pavement. It works with anything that is brightly colored, and large areas of color work MUCH better than small areas.

I have not tried it yet, but I suspect that if you get a pair of the cheap diffraction grating glasses that they sell for watching fireworks displays, you could probably cut them apart and reverse one lens to get the same effect.

Even neater is using the glasses to look at a bowl of rainbow sherbet, letting you eat ice cream in 3D.

Have a wonder-filled week.

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