A level Biology B · Edexcel 9BI0 · Topic 2B
Microscopy
Key points
- Magnification = image size ÷ actual size. Convert units carefully: 1 mm = 1000 µm and 1 µm = 1000 nm.
- Resolution is the ability to tell two close points apart. Light microscopes resolve about 200 nm; electron microscopes much smaller detail because electrons have a shorter wavelength.
- TEM sends electrons through a thin section to give a 2D image of internal detail; SEM scans the surface to give a 3D image. Both need a vacuum, so specimens must be dead.
- An eyepiece graticule is calibrated against a stage micrometer at each objective lens so cells can be measured.
- Stains (such as iodine or methylene blue) add contrast; an artefact is a feature caused by preparation rather than real structure.
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Most cells are far too small to see, so biologists depend on microscopes and on careful unit conversion. You need to calculate magnification, calibrate a graticule and compare light and electron microscopes.
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