A do-it-yourself optical bench

When teaching geometrical optics, especially the parts concerning lenses, there are many experiments that can be done just by holding the lenses in your (or your pupils') hands. But if you want to measure something, for example focal lengths, or combine two lenses to demonstrate the principle of a telescope, then something that can fix the lenses is needed. Of course, we can do such demonstrations and measurements using a commercially available optical bench. But a classical optical bench is usually a large and heavy construction that is not easy to transport. Often only one (or even none) may be available per classroom. In many school experiments something less robust, simpler, and less expensive would perhaps suit the needs better. Here we describe a simple and flexible optical bench that may be constructed by students themselves. Apart from the price of the lenses, the construction is also very cheap. So, if you have some spare lenses available, you can build a small optical bench for $10 or less. Each group of students can then use their own optical bench to do experiments.