Microscopy and Magnification

Magnification $=\frac{\text{image size}}{\text{actual size}}$; convert to the same unit first (1 mm = 1000 µm; 1 µm = 1000 nm). Resolution is the ability to distinguish two close points: light microscopes resolve about 200 nm; electron microscopes about 0.5 nm because electrons have a much shorter wavelength. TEM gives 2D sections with internal detail; SEM gives 3D surface images; both need a vacuum, so specimens are dead. An eyepiece graticule is calibrated against a stage micrometer (divisions of known size, often 10 µm) at each objective. Cell fractionation: homogenise in cold, isotonic, buffered solution, then ultracentrifuge: nuclei sediment first, then mitochondria, then ribosomes.