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Compound Microscope
The simple magnifying glass can magnify an image by a relatively small factor – usually no more than about 5times of the object size. The necessity of more magnification gave the birth the compound microscope.
The compound microscope is designed to provide greater magnification than can be provided by a single lens. The microscope is designed to magnify objects that can be brought close to the device. The magnified image is inverted, but this is usually not a serious problem for most applications.
In its simplest form, the microscope consists of two positive lenses, which are mounted at either end of a tube.
The first lens has a very short focal length, and the object is placed in front of this lens just beyond the focal distance. Since the lens is a positive lens, it forms a real, inverted image at a distance L behind the lens as shown. Since the distance from the first lens to the image is much greater than from the object to the first lens, this image is magnified by the ratio of these two distances. The size of the length L is set by the geometry of the device.
The overall magnification of the compound microscope is defined as the product
of the lateral and angular magnifications:
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