6. Microscopy of phase objects

6.1 Phase object

Some objects are not visible under microscopy because, being perfectly transparent, there is no variation in absorption (or reflection, in the case of reflective objects) in their structure, and so their different zones are not differentiated by any variation in contrast. They do, however, exhibit differences in thickness and/or refractive index between their points. When a light wave passes through such objects, or is reflected by them, it travels along variable optical paths (products of refractive index and thickness), with a longer or shorter path depending on whether the medium is thicker or thinner, at a greater or lesser speed depending on whether the index is lower or higher. These variations in optical path Δ result in deformations of the transmitted or reflected...