An objective with an aperture of 1.4 has accordingly a maximal useful magnification of 1400fold. We assume that the dual nature of light as both wave and particle is known and we will in the following deal with some fundamental principles of wave optics in order to be able to interpret the image formation of a microscope. But our main interest is the understanding of modern techniques of light microscopy like phase-contrast microscopes, polarising microscopes and interference microscopes. The wavelength of visible light is between 400 and 800 nm. The extreme case is given by two waves that extinguish each other.
Diffraction is the partial deflection of a ray of light at the corners of opaque objects.
If a screen is placed behind such a mask and light is sent through, then the pattern of diffraction can be seen. It consists of a regular pattern of points or lines that fades away towards the margins. If a mask with two openings is used, the light is diffracted at the rim of both openings. The diffraction images of the two rays overlie each other and regular enhancements and reductions can be seen in the resulting pattern. The results in the pictures above have already shown that the distances of the points in a diffraction image are reversed proportional to the corresponding distances in the object.