2.3: Light Microscopy (LM)

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Rosanna Hartline
West Hills College Lemoore

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Light Microscopy (LM)

Most microscopes used in college biology laboratories are classified as light microscopes (see the figure, part (a) below) and may also be called compound microscopes since they use two lenses whose magnifications compound (multiply). Visible light passes through the specimen and is bent through the lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living organisms, but since individual cells are generally transparent, their components are not distinguishable unless they are colored with special stains (colored chemicals that makes cells appear to have color such as pink, blue, or purple). Staining, however, usually kills the cells.

Light microscopes commonly used college classroom laboratories are used to magnify specimens up 400 times (400x), but oil immersion lenses can also be used with these microscopes for magnification of 1,000 times (1000x) to view very small structures or very small cells such as bacteria. Two parameters that are important in microscopy are magnification and resolving power. Magnification is the process of enlarging an object in appearance. Resolving power is the ability of a microscope to distinguish two adjacent structures as separate: the higher the resolution, the better the clarity and detail of the image. Due to the magnification limits of light microscopes, in order to gain a better understanding of cellular structure and function, scientists typically use electron microscopes (see image (b) in the figures below) that use electrons (subatomic particles) to carry images instead of light. To see the difference in magnification and detail possible when comparing light microscopes and electron microscopes, compare the images of Salmonella (a type of bacteria) in the figure below.

Side by side images of a standard light microscope on a benchtop and an electron microscope on its station. The light microscope is relatively small in comparison to the electron microscope which has its own control station and stretches from bench-top level to the ceiling.

Above: (a) Most light microscopes used in a college biology labs can magnify cells up to approximately 1000 times (1000x) and have a resolution of about 200 nanometers. (b) Electron microscopes provide a much higher magnification, 100,000x, and a have a resolution of 50 picometers.

In the light microscope image, Salmonella appears as little dots. In the electron microscope image, Salmonella appears as large three-dimensional ovals.

Above: (a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope. (b) This scanning electron microscope micrograph shows Salmonella bacteria (in red) invading human cells (yellow). Comparing Salmonella bacteria when using a light microscope (a), and Salmonella bacteria when using an electron microscope (b), observe the comparative increase in magnification with electron microscopy, the higher level of detail in electron microscopy, and the differences in appearance between light and electron microscopy.

Attribution
"Biology 2e" by Mary Ann Clark, Matthew Douglas, Jung Choi, OpenStax is licensed under CC BY-NC 4.0