The use of microscopes to view ultra-small objects. Optical microscopes use a light source from below or to the side of the object being magnified. For example, in the medical field, there are numerous optical techniques, many of which require altering the samples being observed.
Scanning electron microscopes are able to magnify objects smaller than .2 micrometers, which is smaller than the wavelength of light. Scanning probe microscopes can image down to the molecular level (see AFM