An increased interest in physical optics started about 1930. The paper gives a personal selection of some special topics. First, light rays are shown to be different from the paths of the energy. The diffraction patterns of lens errors cannot even approximately be found from the geometric patterns. The new concept of partial coherence is treated next. It has been made possible by introducing a less stringent definition of incoherence. Total reflection causes an absolute phase shift strongly dependent on the angle of incidence. This explains a ray displacement predicted from wrong premises by Goos and Hänchen. Even the old relative phase shift, as in the Fresnel rhomb, gives rise to a new effect, namely, a destructive interference between the two halves issuing from a roof prism. Phase differences which are otherwise unobservable become visible on a coherent background, which is obtained in a simple and accurate way by diffraction.
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