While most people picture the compound model from lab class when they think of microscopes, many types of microscopes are actually available. These useful devices are employed by researchers, medical technicians and students on a daily basis; the type they select depends on their resources and needs. Some microscopes provide greater resolution with lower magnification and vice versa, and they range in cost from tens to thousands of dollars.
The simple microscope is generally considered to be the first microscope. It was created in the 17th century by Antony van Leeuwenhoek, who combined a convex lens with a holder for specimens. Magnifying between 200 and 300 times, it was essentially a magnifying glass. While this microscope was simple, it was still powerful enough to provide van Leeuwenhoek information about biological specimens, including the difference in shapes between red blood cells. Today, simple microscopes are not used often because the introduction of a second lens led to the more powerful compound microscope.
With two lenses, the compound microscope offers better magnification than a simple microscope; the second lens magnifies the image of the first. Compound microscopes are bright field microscopes, meaning that the specimen is lit from underneath, and they can be binocular or monocular. These devices provide a magnification of 1,000 times, which is considered to be high, although the resolution is low. This high magnification, however, allows users to take a close look at objects too small to be seen with the naked eye, including individual cells. Specimens are usually small and have some degree of transparency. Because compound microscopes are relatively inexpensive yet useful, they are used everywhere from research labs to high school biology classrooms.
The stereo microscope, also called a dissecting microscope, provides magnification of up to 300 times. These binocular microscopes are used to look at opaque objects or objects that are too large to be viewed with a compound microscope, since they do not require a slide preparation. Although their magnification is relatively low, they are still useful. They provide a close-up, 3-D view of objects’ surface textures, and they allow the operator to manipulate the object during viewing. Stereo microscopes are used in biological and medical science applications as well as in the electronics industry, such as by those who make circuit boards or watches.
Unlike stereo and compound microscopes, which use regular light for image formation, the confocal microscope uses a laser light to scan samples that have been dyed. These samples are prepared on slides and inserted; then, with the aid of a dichromatic mirror, the device produces a magnified image on a computer screen. Operators can create 3-D images, as well, by assembling multiple scans. Like the compound microscope, these microscopes offer a high degree of magnification, but their resolution is much better. They are commonly used in cell biology and medical applications.
Scanning Electron Microscope (SEM)
The scanning electron microscope, or SEM, uses electrons rather than light for image formation. Samples are scanned in vacuum or near-vacuum conditions, so they must be specially prepared by first undergoing dehydration and then being coated with a thin layer of a conducive material, such as gold. After the item is prepared and placed in the chamber, the SEM produces a 3-D, black-and-white image on a computer screen. Offering ample control over the amount of magnification, SEMs are used by researchers in the physical, medical and biological sciences to examine a range of specimens from insects to bones.
Transmission Electron Microscope (TEM)
Like the scanning electron microscope, the transmission electron microscope (TEM) uses electrons in creating a magnified image, and samples are scanned in a vacuum so they must be specially prepared. Unlike the SEM, however, the TEM uses a slide preparation to obtain a 2-D view of specimens, so it's more suited for viewing objects with some degree of transparency. A TEM offers a high degree of both magnification and resolution, making it useful in the physical and biological sciences, metallurgy, nanotechnology and forensic analysis.