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L7-21: MAGNIFYING GLASS - TV 9 years 11 months ago #941

  • zzfixk21
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ID Code: L7-21
Purpose: Illustrate 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 rule brought into focus as viewed on the monitor (photograph at left above). 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 (photograph at right above). The image size is virtually 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 (photograph below). The focal length of the magnifying glass below is 10cm, one-half of that for the maginfying glass above.

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.

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