Smoking in School

A Freznel Lens is a very clever little invention. It's a way to make a "fat" lens thin. A fat lens you see will have a short focal lens and sometimes (lighthouses and overhead projectors) that is desired. Unfortunately large fat lenses are very heavy because they are, well, FAT! So what is done is to just copy the shape of the lens at many radii and stick that shape on the flat sheet.



You see these on the underside of the glass plate on overhead projectors and sometimes in back windows of vans and such. I also see them as magnifiers to help people read books with small print. Well, I had one knocking around my lab and for fun I taped it to the window in my room. When you look through it then everything far away is upside down. Just a nice little curiosity and I liked having those things sprinkled about the room.

One day I was lecturing away (fascinating as always I'm sure!) and a student was frantically waving his hand. Bathroom? Not really a good time for a question so I looked away. His hand did not come down and was very insistent.

OK, "What is it?"
"Mr. Welty, the curtains are smoking!"

They were. My windows face east and in the morning I'd close the curtains to keep out the glaring sun. Unfortunately the curtains were 1 focal length away from the lens. Burned a hole right through! Funny. . . unless it had been Saturday!