How a Compound Light Microscope Works
To comprehend how the compound light microscope (also called a "bright field" microscope) works we must first understand that convex lenses bend light rays in a peculiar manner1 so that light hitting the center of the lens goes straight through. But light hitting other areas is bent toward a focal point. This bending allows the view at a specific distance from an object to see the image as larger than it would appear to the naked eye.
In the 1590s Zaccharia and Hans Janssen came up with the idea of lining up two convex lenses. The image at left shows this principle in action. Light is first emitted by the light source and is directed by the condenser lens on to the specimen, which might be a loose object, a prepared plate or almost anything. A microscope can even be applied to small parts of larger objects, though with a bit more difficulty. (The light does not absolutely need to originate below the specimen.)
The light from the specimen then passes through the objective lens. This lens is often selected from among three or four and is the main determinant for the level of magnification. It bends the light rays and in the case of this example sends them to a projector lens, which reverses their direction so that when the image reaches the eye it will not appear "upside-down". Not all microscopes have a projector lens, so the viewer may be seeing a reverse image. In these cases, when the slide is moved, it will appear to be moving in the opposite direction to the viewer.
The light rays then travel to the oracular lens or "eye piece". This is often a 10X magnification lens, meaning it magnifies the magnified image an additional ten times. The image is then projected into the eye. It is very seldom that a specimen is in focus the moment it is placed beneath a microscope. This means that some adjustment will have to be made. Unlike in telescopes, the focal length between lenses remains constant when adjusting the focus. The lens apparatus is brought closer to or further from the object. The focus adjustment is often along the neck of the tube containing the lenses, but it might just as well move the slide up and down. The best way to make this adjustment is to make a course adjustment so that it is too close to the object and then back off with the fine adjustment2. This helps to ensure that the specimen is not inadvertently smashed by the lens.
A microscope may have either one or two oracular lenses, it may even project the image on a computer screen. With binocular microscopes, the eye pieces will often need to be adjusted for each person who uses the device. The distance between eyes will be compensated for by an adjustment that moves the lenses closer together or farther apart. Difference in the strength of vision between individual eyes may also be adjusted for at the eye-pieces.
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