Physics 2000 Einstein's Legacy CAT Scans

Projecting Shadows

How can you get a picture of a slice of something without cutting it apart?

To keep it simple at first, think about this: what can we learn about the shape of something just by looking at shadows? Let's shine spotlights on some different shapes and look at the shadows they make:

Click on the spotlights to turn them on and off.
Choose different shapes from the menu.

(By the way... There's something not quite right with the lights and shadows here. Do you know what it is? Did you notice it before?
If you can't spot it, here is the answer.)

I can't tell a whole lot, except for how wide the shape is in the direction the light is shining on it.

That's a start. Try this:

  1. Turn on only the left and bottom lights; make sure the diagonal light is off.
  2. Pick 'Square' from the menu, and look at the two shadows carefully.
  3. Now pick 'Circle' from the menu, and compare the shadows. You may want to switch back and forth a few times. Notice how the shadows are the same?
  4. Now turn the diagonal light on, and compare the shadows from the square and circle again.

Oh, I get it. Each direction we look at the shadow tells us something more. Does that mean that the more ways we look at it, the more we know about it?

Exactly. That's why real CAT scan machines go around in a circle: so they can take x-rays from all sides.

Um...wouldn't it be easier to just spin the shape and watch the shadow change?

It would give us identical results. Of course, the most we could ever learn using plain old visible light is what the outside contours of something are. But x-rays enable us to do a similar trick on the inside.

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