What are the optical properties of transparent spheres? Or: What do you see when you look into a crystal ball?
To find out, I wanted some decent-sized glass globes, so that I could have my students look through them, and I put a fair bit of effort into trying to locate appropriate crystal balls. To my disappointment, all of the samples I found were unusable. They either had markings or decorations of some kind, or were coated with some kind of hazy gloss, or were prohibitively expensive. I just wanted a perfectly clear glass sphere, and I couldn’t find any. But I did eventually find a couple of good substitutes–both simple, cheap, and effective.
Snow Globes and Marbles
One day while wandering through the craft store, I discovered that they sell components of snow globes, for people who want to make their own. Normally, you buy the hollow plastic globe, a snowman or something to put inside, and some snow. Then you fill the globe with water, assemble it, shake it, and watch the snow settle. But if you just buy the globe and fill it only with clean water, you have your very own crystal ball.
(Assembly tip: You may find it difficult to fill the globe without trapping air bubbles inside if you pour water into it from a faucet or something. You can make the globe bubble-free by submerging it in a sink or bucket and assembling it underwater.)
Another solution I discovered is decorative marbles. I originally gave up on marbles, because I had looked in toy stores, and all of their marbles were colored or had decorations of some kind. Even the “plain” ones often had a hazy sheen that made them unusable as transparent spheres. But I later discovered that craft stores sell small, plain, clear marbles in large quantities for filling vases and other decorative purposes. These make wonderful classroom crystal balls because you can buy large numbers of them cheaply, and then every student can take one home (if they can keep from losing it before the end of the day.) The only flaw is that they often contain air bubbles, but those aren’t usually too distracting. They are also tiny, but the kids don’t seem to mind that too much.
Another suggestion that I have come across is to bend the end of a paperclip into a loop, and try to capture a tiny droplet of water. This might be fun for students to try, but in my experience, it only really works for extremely tiny droplets–too small to be useful.
Playing With Crystal Balls
So, to come back to my original question, what do you see if you look through a transparent sphere, or a crystal ball? The first thing students usually notice is that everything is upside down. You can see this clearly in the pictures above. I also like to point out that everything has shrunk. If you were to look through a plain flat disk of the same size, you would see a disk-sized portion of the world on the other side. But the sphere has taken the entire scene on the other side, from one horizon to the other, and squashed it into the face of the sphere. In a sense, you can see the whole world in a crystal ball.
If you place the crystal ball up close to something, you will also notice another power, or ability, of the sphere–it can magnify things.
As you play around with transparent globes, especially when you bring them close to surfaces in order to magnify them, you may also notice another interesting ability of crystal balls. In the second photo above, you may notice images of the lights in the background appearing on the desktop. The globe has made a picture of the lights appear on the surface of the desk. (The photo quality isn’t that great. The images were much clearer in real life.) You can notice the same thing if you hold up a globe and a “screen” of some kind in a dark room with a TV or a window or some other bright picture on the other side. The picture below shows a globe projecting a picture of a window onto the nearby wall. So here is another ability of transparent spheres: you can cast pictures of bright scenes onto screens.
So, to summarize some interesting abilities of transparent spheres:
- They make far away things appear small and upside-down.
- They magnify very close things.
- They can cast images of bright scenes.
Can you think of any objects with similar powers? In a way, magnifying lenses are just more refined and more carefully shaped versions of crystal balls. Do magnifying lenses have all the same abilities as crystal balls?