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Science, Maths & Technology

Scanning electronic microscope

Updated Thursday 3rd August 2006

How the scanning electronic microscope allows us to see ultramicroscopic objects so clearly

Woman looking into microscope Copyrighted image Icon Copyright: BBC

The SEM is the only tool we have that allows us to view ultramicroscopic objects much as they would appear if we could see them directly with our eyes. Because electrons are so much smaller than the photons of visible light, they can be harnessed to magnify objects to a much higher degree than optical microscopes can (up to the hundreds of thousands), not only with great clarity but also with the depth of focus that gives SEM imagery its characteristic three-dimensional quality.

The SEM uses a fine beam of electrons controlled by electromagnetic lenses, focused to a point that scans across the specimen. The impact of the beam liberates electrons from the surface of the sample that are used to recreate its image on a viewing monitor. Traditional SEM's like the older Cambridge 250 that produced these micro-graphs require dry samples isolated in a vacuum chamber, but newer technology allows wet samples to be imaged as well. 

Electrons can only yield black and white images, and any colour is applied artificially after the fact. Monochromatic blues or sepias can be produced by using particular types of film, while multiple colours can be applied by hand or with a computer. Depending on their ultimate use, some micro-graphs are colour-enhanced in natural hues, while others range farther afield for a more exciting effect.


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