The microscope: magnification & resolution

Cells and the structures inside them are far too small to see with the naked eye, so biologists use microscopes. In this lesson we learn the principles of microscopy — how a light microscope uses light and glass lenses, and how an electron microscope (both TEM and SEM) uses a beam of electrons and electromagnets. We draw the crucial distinction between magnification (how many times larger the image is) and resolution (the smallest distance at which two points are still seen as separate), and see why electrons resolve so much more detail than light — their wavelength is far shorter. We get comfortable with the units millimetre, micrometre and nanometre and how to convert between them, then we do the calculations the examiner loves: magnification equals image size divided by actual size (the I, A, M triangle), and finding the actual size of a specimen from a drawing, photomicrograph or electron micrograph. Finally we calibrate an eyepiece graticule with a stage micrometer and use it to measure a real cell.