For scientists attempting to delve into the mysteries of the tiniest living things, the naked eye is simply not powerful enough. Unaided, the human eye can distinguish objects of about 0.1 millimeters (0.004 inches) in length; this is strong enough to see a human egg cell, but not most bacteria or any viruses. Microbiologists use different types of microscopy to get a better look at these miniscule forms of life.
Shedding Some Light
The compound light microscope improves on the resolving power of the naked eye, or its ability to distinguish separate objects, by a factor of roughly 500; a good microscope can distinguish objects separated by as little as 0.2 micrometers (0.0000079 inches). Light microscopes operate by illuminating an object and then focusing the light reflected off of that object through a series of lenses. Depending on the lenses used, the resulting image is also magnified, usually in a range from 100X to 1000X the object's actual size.
A Healthy Glow
Fluorescence microscopy also operates using visible light, but instead of employing reflected light, it uses light emitted by the object itself, or fluorescence. The object of interest is often stained with a compound, called a fluorochrome, that gives off light of a specific color when excited by light with a different wavelength. The microscope then filters out the wavelength of the light used to excite the fluorchromes, allowing the researcher to see only the parts of the object that are emitting fluorescence.
Viewing transparent objects with a standard light microscope can be difficult, but phase-contrast microscopy cleverly uses the differences within a sample to obtain a better image. As light passes through different materials, such as the cell wall and nucleus of a plant cell, it travels at different speeds due to the different refraction indexes of each material. A phase-contrast microscope is constructed so that highly refractive parts of the object appear much darker than its unrefractive parts, defining its details.
Getting Even Smaller
For the smallest objects, such as viruses and individual atoms, researchers must turn to electron microscopy: this technique can resolve objects just 0.10 nanometers (0.0000000039 inches) apart. Two types of electron microscopes exist. The transmission electron microscope sends a beam of electrons through a very thin slice of an object, and the deflection of that beam is then used to reconstruct the object's image. Scanning electron microscopy, on the other hand, uses the electron beam to excite the emission of secondary electrons from the surface of the object, which are then used to create the image.
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