The Random Destructive Acts FAQ

Updated March 19, 2003:

It has been about 8 years since I wrote this page (before 2002 the last modification date was June 30, 1995) and I still get emails about it every few days. The most frequently asked question is, "Where can I buy one?" Edmund Scientific quit making this rectangular lens a few years ago and the company split, but Edmund Industrial Optics now makes a round 35 inch diameter lens (and many smaller sizes). Last I checked it was on this page but that's likely to change. It was part number NT43-921: FRESNEL LENS 35.0" DIA and cost $224.10.

Jacob Dickinson informed me that Alltronics.com sells a rectangular lens similar in size to the infamous lens we had, and a few weeks ago I got one (see below). It's thinner than the Edmund's lens we used to have and needed a frame, but it's big and about half the price of Edmund's lens. You can find it at http://www.alltronics.com/lenses.htm and it was $99 plus shipping when I looked.

If you're looking for something smaller and a lot cheaper, you can often find cheap surplus (used) 12" Fresnel lenses at American Science and Surplus and Scientifics (the new owner of the Edmund Scientific Catalog) for as little as $3.

And to answer another only slightly less frequently asked question, no the beloved Dershem Memorial Lens is not for sale. We decided it would go to the first of us to get a Ph.D. (in other words, to first order we left it up to fate) and Chris Chaput was the proud winner. Thus Dr. Chris now owns the lens and (for obvious reasons) it is kept at least 1000 miles away from him at all times.

People also ask, "What was the area of the focus?" The best we ever got was about a centimeter across. The lens was somewhat flexible so the corners didn't tend to contribute much. A professor at U of M who had one of these said he built a frame with cross supports and was able to get more of the lens to contribute to the focus.

A lot of people want to know what temperatures we achieved. It's hard to make this estimate, the real question should be how much power the lens can supply (and you can see more about this below). But in terms of estimates, we certainly melted aluminum which has a melting point of 660.37 degC. We never quite managed to melt copper with the old lens but (melting point of 1083.4 degC) but we have with the new one (see below) so I think we could have if the sidewalk had not been so fragile. Assuming a black body target and that the sun supplies 1340 W/m^2 (before atmospheric absorption) we calculated that we should be able to melt quartz (1610 degC) but in practice I don't believe we achieved temperatures that high.