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How Light Microscopes Magnify Objects
& Are Limited by Resolution
from Science Prof Online
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Light waves passing through a lens bend, or refract the light. Because lenses are curved, light that passes through the edge of the lens bends more than the light that passes through the center of the lens. This makes the light spread out after it passes through the lens, producing an enlarged, and inverted (upside down and backwards) image of the specimen. How much the specimen is enlarged depends on the thickness of the lens, how curved the lens is, and how fast light moves through the object being viewed.
How Much Can a Light Microscope Magnify an Object?
A light microscope can clearly magnify a specimen to 1000x its actual size, when an oil immersion lens is used. For some objects, such as viruses, which are typically 100nm or less in size, and the detailed ultrastructure of living cells, 1000x is not adequate magnification.
Relative Size of Microscopic Objects
Most people are, understandably, accustomed to measurements of things that are visible to the naked eye…meters, centimeters and even millimeters. Micrometers and nanometers are units of measurement that belong solely to the microscopic world.
There are one thousand millimeters, or one hundred centimeters in a meter. The smallest object that the unaided human eye is capable of seeing is a fraction of a millimeter.
In one millimeter, there are a thousand micrometers, represented as µm (µ is the Greek lowercase letter mu). The eukaryotic cells of animals and plants are tens to hundreds of micrometers in size. Bacteria are typically just a few micrometers in size. Viruses are even smaller - mere nanometers; the next unit of smallness in the world of microbiology, with a million nanometers making up one millimeter. It is in the realm of nanometers that light microscopes lose their power to reveal clear images.
Resolution Limits the Magnification of a Light Microscope
With combined lenses, light microscopes would be able to magnify objects millions of times, but the image would be blurry - unresolved. Resolution is the ability to distinguish between objects that are very close together. In other words it relates to the clarity or amount of detail one can see when viewing an image, and resolution is limited by the wavelength of radiation used to view the specimen.
If the objects of interest in the specimen are much smaller than the wavelength of the radiation being used to illuminate them, they do not interfere with or interrupt the waves, and so are not visible. Because the minimum wavelength of visible light is 400nm, light microscopes cannot distinguish between objects closer together than 200nm. In order to clearly see microscopic objects smaller than 200nm, a type of radiation with shorter wavelengths must be used, such as the beam of electrons used in electron microscopes.
Sources
- Bauman, R. (2007) Microbiology with Diseases & Taxonomy, Pearson Benjamin Cummings.
- Biology Mad web site (2004).
- Tortora, G., Funke, B., Case, C. (2010) Microbiology, an Introduction. Benjamin Cummings.
Article Summary: Light microscopes pass waves of visible radiation through lenses in order to increase the apparent size of the object being viewed. Here's are the basics.
How Compound Light Microscopes Magnify Specimens
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100x Objective Lens of compound light microscope provides maximum total magnification of 1000xTM.
Page last updated: 2/4/2012
This article originally appeared on Suite101 online magazine.
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