PURPOSE: To illustratre how a magnifying glass 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. First the focus is set at the near point of the TV lens (the near point of the eye), approximately 20 cm, and the ru le brought into focus as viewed on the monitor. The 20 cm focal length convex lens magnifying glass is then inserted as shown in the photo, the focus adjusted to infinity, and the object position adjusted slightly to obtain the best focus. The image size is the same! This demonstration illustrates that the function of a magnifying glass is not to "magnify" but to allow the eye to relax, or focus at infinity, while viewing an object from very close, so that it intercepts a larger angle and looks bigger.

Magnification is obtained by using a magnifying glass that has a focal length much shorter than the near point of the eye. The magnification M=f(eye)/f(lens), where f(eye) is the near point of the eye. The object to be viewed is located at the focal point of the magnifying glass, which is held close to the eye.

The eyepiece on most optical instruments, such as telescopes, microscopes and binoculars is a magnifying glass.



EQUIPMENT: TV camera, 20cm, 10cm and 5cm focusing lenses, and mounted rule on optical rail, TV monitor not shown.

SETUP TIME: 5 min.

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