How Do Researchers See Viruses?
Though 800 times smaller across than a human hair, the HIV particle is larger than most other viruses. Even so, it was very challenging to discover what the HIV virus looks like and how it is constructed. You cannot observe a virus particle on the stage of a normal optical microscope, which works with visible light and has a practical limit for magnification.
An optical microscope’s diffraction limit, or resolution (ability to separate two closely spaced objects) is based on the wavelengths of visible light, which range from about 400 to 700 nm (violet to red). The minimum practical resolution (or distance between two objects) is less, about 200 nm. Any specimens closer together than 200 nm appear as a single object under an optical microscope. Consequently, the useful magnification power of optical microscopes is limited to approximately 1,500x. Pushing to a magnification power higher than that leads to hopelessly fuzzy images that are impossible to resolve clearly. Thus, an HIV particle, which measures 120 nm across, is smaller than optical microscopes will allow us to view, even at maximum resolution.
Because virologists (scientists who study viruses) must be able to “see” objects as small as a single nanometer, they require microscopes with much greater magnification power. However, “seeing” is not quite what they do. Rather, they employ a variety of sophisticated microscopes that create images on a computer screen.
One such instrument is the transmission electron microscope, or TEM, which directs a beam of electrons through a very thin specimen. The electrons interact with the specimen and are shifted slightly as they pass through. Then, they fall onto a fluorescent screen or a detector, similar to a CCD chip in a digital camera, where the TEM’s image is created. Typically, electron microscopes are able to produce useful magnifications one million times the actual size. But under special circumstances, 50 million times magnification has been achieved.
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- Vogt, G., and Moreno, N. (2012) The Science of HIV/AIDS Teacher’s Guide. Baylor College of Medicine: Houston. ISBN: 978-1-888997-62-0
- Photo courtesy of the National Center for Macromolecular Imaging © Baylor College of Medicine.
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Grant Number: 5R25RR018605