PURPOSE: To illustratre how a Galilean telescope works.

DESCRIPTION: The TV camera functions as the eye, so what the eye sees can be viewed on a monitor or the rear projection screen. A 10 cm or a 5 cm concave lens acts as the eyepiece and a large 20 cm convex lens is the objective lens, with the TV camera focused at infinity (relaxed eye). In this case the image of the objective lens is on the left side of the concave eyepiece - behind the eyepiece. The rule is positioned across the lecture hall (at infinity) from the telescope. The image is seen to be erect and to have a magnification M=f(ob)/f(eye). The magnification can be checked by viewing a two-meter stick with the TV camera directly and then with the telescope. Compared with the astronomical telescope, the image is more faint and has a smaller high-quality field of view.

The images below show the object rule viewed directly by the camera "eye," the telescope image with a 10cm fl eyepiece (M=2) and the telescope image with a 5cm fl eyepiece (M=4).

To set up in practice, set the camera focus and lens positions and move the objective lens until the image is seen by the camera.

SUGGESTIONS:

See Question of the Week #124 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: TV camera, 5cm and 10 cm concave lenses and large 20 cm convex lens on optical rail, and mounted object on separate optical rail, with TV monitor.

SETUP TIME: 5 min.