Introduction
As indicated by its etymology (micro‑ [ \(\mu \iota \kappa \rho \acute{o} \varsigma\) small ] and ‑scope [ \(\sigma \kappa o \pi \epsilon \ddot{\iota} \nu\) observe ] ), a microscope is an instrument allowing a visual observation of very tiny objects or object details located in a close vicinity of the observer, that would be indistinguishable to the naked eye even when placed at the punctum remotum. Hence, similarly to a magnifying glass, an essential property of a microscope is its magnification, which represents its aptitude to deliver an image angularly magnified of the observed object. However, this parameter is not sufficient to fully characterize the instrument performances. Indeed, this property has to apply to all objects, including the smallest ones. Therefore, the second key property of a microscope is its separating or resolving power[1]. In practice, this latter point has very important consequences, since it requires workink with diffraction-limited optics of very large numerical apertures (i.e. without aberrations).
Even though optical microscopy is a very ancient science that took off in the XVIth century, it still remains an advanced technology, widely used in numerous research and industry areas, with a lot of technical applications such as the extension to the UV range, or the availability of digital photomicrography easy-to-use systems.
This first unit of the microscopy course aims at presenting in details the basic concepts of instrumental optics (geometrical optics and diffraction), on which are based all microscopy systems. A second unit more specifically presents the numerous techniques dedicated to improve the contrast of the observed objects.