Microscopes: From Light to Electron
Overview: The Microscope
The microscope is a valuable tool in biology and other sciences. It allows investigators to study many objects that are too small to be seen with eyes alone. While many observations can be done with simple and compound light microscopes, the smallest details are shown with electron microscopes.
Simple Light Microscopes
The simplest microscope is a magnifying glass that works by focusing light to magnify very small objects. Scientists may have used them as early as the 15th century, although the finest lenses were made by van Leeuwenhoek in the late 17th century. He was able to study and draw organisms in pond water, a very familiar activity in science classes from elementary school on.
Compound Light Microscopes
The earliest compound light microscope had a lens on each end of a long tube. They worked by using one lens as a magnifying glass, then magnifying that image with another lens. Scientists tended to prefer single lenses for magnification, because the images from early compound light microscopes were distorted. Modern compound light microscopes minimize some of the distortion by the way the lenses are arranged.
The most common microscopes used in science classes are compound light microscopes. One lens is near the eyepiece of the microscope, and the other lens is positioned near the specimen slide. The light source is under the slide, so that light actually travels through the slide and specimen and then is focused through the lenses. The increase in the specimen’s size is measured by magnification. The power of the lenses is multiplied as they work together, so an ocular lens in the eyepiece that magnifies 10 times and an objective lens that magnifies the size of the object 43 times will magnify the entire specimen 43 X 10, or 430 times its size. At higher magnification than 2000 times, images become too distorted and fuzzy and lose resolution.
Many specimens must be specially prepared for viewing under a microscope. Tiny slices of matter must be thin and small enough to fit on the microscope slide. A microtome is a tool that cuts very thin slices from specimen material. In addition, specimens must often be stained in order to show up details. Common stains color red or blue, and special dyes have been developed that do not kill or distort cell structures. Specimens for the electron microscope must be embedded in plastic, cut even thinner, and coated with a very thin metallic layer.
Electron microscopes produce a source with a wavelength smaller than visible light, so objects can be magnified much more than 2000 times their size. However, specimens are prepared differently and are viewed in a vacuum. The transmission electron microscope magnifies specimens more than 200,000 times their size, and a scanning electron microscope produces highly-detailed, three-dimensional images. Electron microscopes have been developed that combine the best features of the transmission electron microscope and the scanning electron microscope.
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