Microscopes: Introduction


In biology, there are lots of things that are interesting but too tiny to see well - sometimes to see at all - with the naked eye. A magnifying lens is good, but a series of lenses, each magnifying the image of the last, works better. That's a microscope.

Anything that you want to look at with a microscope can be called your specimen, and the nature of your specimen may dictate what sort of microscope you need to see it with. If you are looking at the outside, you want a scope that scans the surface - a scanning microscope images light (or other radiation) reflected off the surface of a specimen. To look inside, you need to get light (or other radiation) to go through your specimen - it needs to be thin, semi-transparent, or both. A microscope that sees images passed through a specimen is a transmission microscope.

Light microscopes, which you'll be using, use light to produce their images. Scanning light microscopes are called dissecting microscopes, and there are many kinds of light-transmission 'scopes, named for the way the light is delivered. Our lab has mostly bright field compound microscopes - specimens are seen against a bright background, and several (compound) magnifications can be chosen by rotating the objective lenses. The magnification of the objectives - commonly 4X, 10X, 40X, and 100X - are magnified again by the eyepiece or ocular lenses - usually 10X - so the total magnification for the different objectives are 40X (4x10), 100X (10x10), 400X (40x10), and 1000X (100x10
).  In any microscope system, the total magnification is calculated by multiplying the lens powers together.  In many electron microscopes, there are whole series of lenses, which helps to make them very powerful.

But magnification isn't the most important feature of a good microscope - making things look big is good, but not if you can't see them clearly. The ability to focus clearly, technically to tell that two close-together objects are separate, is called the resolution of the microscope, and with even the best light 'scopes it "tops out" at about 1200X, because of the nature of light beams themselves. Looking at really tiny things requires really tiny beams, and that's why you'll often see pictures of tiny objects (even atoms!) taken with electron microscopes (abbreviated EM - SEM for Scanning Electron Microscope and TEM for Transmission Electron Microscope). Those 'scopes use adjustable electron beams, focused with magnets and made visible by TV-type screens.



Step-By-Step Instructions




First Written 1988;  Web Version 2001;  Last Update 2012,  M. McDarby




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