10. Optical Instruments

A set of optical instruments is available. Most of them are in a box which you can keep at the front of the class for the students to come down and look at. You have to sort of watch over them to make sure none get dropped or lost. Or you could pass one at a time around and carefully collect it afterwards.

There are opera glasses, which consist of a pair of Galilean telescopes made from a diverging eye lens and a converging objective. This arrangement produces an erect image with only two simple lenses, but at the expense of low power and small field of view. Prism binoculars have converging eye lenses, probably two elements each, with prisms to fold the optical path and erect the image. A pair of Porro prisms is available to show how the inversion of the image is effected, and also a roof prism set is available to show how the same job can be accomplished "in line". A small astronomical telescope produces a bright, clear, wide field image, but upside-down. There is also a "Captain's telescope" with telescoping tubes and an internal converging lens to erect the image. This arrangement produces an erect high power image, but the image is dim and difficult to focus, and the instrument has an unwieldy length.

You can show a microscope and a slide of the lens system of microscopes. Art has a transparency diagram of the elements of the human eye, and there also is a slide of an actual cross-section of an eye. Finally, a demonstration camera with a large plano-convex lens will produce images of lights, etc. in the lecture hall, with some distortion apparent because of the simple lens.