Optical Microscopes vs. Electron Microscope

Difference Between Optical Microscopes And Electron Microscope

Prior to 1930, all microscopes were “optical”: they produced images through the utilization of visible light—the form of electromagnetic radiation that the human eye can see. The first and most simple optical microscopes were hand lenses, also known as magnifying glasses. These generally consist of a single biconvex lens. Magnifying glasses are capable of forming images that are as much as 20 times larger than the actual size of the object being viewed. The amount or power of magnification taking place is customarily expressed as a number followed by a times sign (×). For example, the best magnifying glasses are capable of 20× magnification, or 20 times the power of the naked eye. This expression is used in reference to both the image being magnified and the lens doing the magnification.

Having only a single lens system (a system that might consist of one or more lenses joined together), magnifying glasses can offer only one stage of magnification. This greatly limits the power of magnification. Having but a single lens system also makes magnifying glasses susceptible to inherent defects or aberrations in the lens that can result in distorted images. One common aberration is astigmatism, in which an image is made fuzzy by light rays that pass through the lens obliquely (at an angle) instead of parallel to the axis of the lens, and consequently do not converge at the principal focus. Even light rays that do pass through the lens parallel to the axis can miss the principal focus, especially when they strike the outer edge of the lens. This is caused by a spherical aberration in the curvature of the lens, and it too results in a distorted or imperfect image.

Electron Microscope

In providing a closer look at things, compound optical microscopes have greatly expanded our scientific knowledge. However, so long as only visible light was utilized, the look was not close enough. Visible light waves can resolve only specimens that are larger than the waves themselves. Since the average length of a visible light wave is 1/50,000 of an inch (0.005 millimeters), or 5,000 angstroms (one inch equals 254 million angstroms), a visible light wave is the smallest specimen a compound optical microscope can “see.” This means that the most powerful optical microscopes can barely resolve images of many bacteria and viruses, and they cannot be used at all to study atoms or molecules, which generally measure only a few angstroms in diameter.


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