Burning Diopters

I’m not going to get technical or use math in this post but want to explain how a magnifying glass works but before I do that there’s a little history to magnifying lenses. I won’t go over what’s already on the internet about this but you might have wondered how people started fires hundreds of years ago or even thousands of years ago. It’s not the only way fires were created long ago, I’m sure the fire starter experts know all the other ways. Let’s just say that the modern world of optics started when someone found that piece of rock that when held at the right distance from some dried plant material caught it on fire. Can you imagine being the first person in the whole world to pick up a rock and see that it can start a fire! Who knows when that happened. I’m just guessing, maybe 5,000 or 10,000 thousand years ago (not exactly modern times I guess). A magnifier designed to do this is also called a burning glass or burning lens. I did read on wikipedia that people used vases filled with water to focus light and thus could start a fire with that. I’d like to see how those were used. Here’s a link to modern day fire starting with plastic bottles.

A magnifying lens makes something larger than it really is. Now this is magic! But not really because we all know what a magnifying lens does but maybe not exactly how it works. Let’s take this one example of how it works in reverse. If you hold the magnifying lens at the right distance and try to burn something using the suns rays you are making the big sun in the sky very small onto whatever you’re trying to burn. And what is really happening is you’re concentrating all the light from the sun into a very teeny small area and all those concentrated rays make a lot of concentrated energy. You might wonder how rays can be compacted (but really called focus). Well, that’s just something that is part of the laws of nature.

If we take that same experiment and instead of the suns rays going through to the piece of something you’re burning, put your head in front of the magnifying lens and look at that little piece of something you were burning. The magnifier works both ways you could say. Dual purpose. It really depends on how you use the magnifier. You probably use it to look through so you magnify what you’re looking at instead of making fires I hope.

I know I said I wasn’t going to do any math in this post and so far I haven’t. But let’s look at a couple of things about a magnifying lens that make them work. First, its thicker in the center and thinner on the edges. Some people’s glasses are made this way. I wrote about farsightedness and presbyopia recently. Both of those crowds use these kind of lenses to converge rays so they can see clearly. Yes, converging in this case means bring the rays closer together (as opposed to diverging rays in a nearsighted person’s glasses). This type of converging lens also has what we call plus power and is measured in diopters (that’s basically what your eyeglass prescription numbers are all about). I believe I read that the origin of the word diopters came from Johann Kepler who lived around the 1600’s. Yes, he’s mentioned in every astronomy book I’ve read (and, yes, I do love astronomy).

A magnifier usually isn’t measured in terms of it’s dioptric power though. It’s measured in how much larger it can make images appear. There are formulas to calculate this but I said I wasn’t going to do any math. Over the counter reading glasses do magnify a little somewhat. I’ve heard some people call those glasses, “magnifiers.” But true magnifying lenses have so much dioptric power than you’d have to put something just inches away from your face to read and to use them as reading glasses. And that’s not comfortable. This is why you put magnifiers inches from what you’re looking at to make them work instead of inches from your eyes.

Confused? Hopefully not too much. Wait until I write a post on virtual images.