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Optical Instruments

By | November 2010
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Optical Instruments

A microscope
A basic microscope is made up of two converging lenses. One reason for using two lenses rather than just one is that it's easier to get higher magnification. If you want an overall magnification of 35, for instance, you can use one lens to magnify by a factor of 5, and the second by a factor of 7. This is generally easier to do than to get magnification by a factor of 35 out of a single lens. A microscope arrangement is shown below, along with the ray diagram showing how the first lens creates a real image. This image is the object for the second lens, and the image created by the second lens is the one you'd see when you looked through the microscope. Note that the final image is virtual, and is inverted compared to the original object. This is true for many types of microscopes and telescopes, that the image produced is inverted compared to the object.

Cameras and Projectors
The camera is essentially a dark box (camera oscuro = dark room) into which light is passed through a lens, forming a real image on the back wall where a light sensitive substance (film or photoelectric material) is placed.

The camera is “focused” (so that the image forms in the right place) by manipulating the lens, either by moving it back and forth or by changing its focal length. Usually the object is beyond 2f distance, and the image distance is between f and 2f, so the image is reduced. Special close-up cameras can focus on objects at 2f or closer. The projector is a camera in reverse. The object is an illuminated transparency, placed between f and 2f distance. The image is formed on a screen at distance greater than 2f, so it is enlarged (and inverted relative to the object).

The Simple Magnifier
Under ordinary daylight conditions the limit of a normal eye in resolving nearby objects is about 1 minute of arc, or about 0.0003 rad. This is called the acuity of the eye. If two point objects are at the near point (25 cm) they...

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