Modern optics of long-wavelength neutrons

This paper is dedicated to the memory of Academician Il'ya Mikhailovich Frank (1908–1990). The papers of I.M. Frank for the period 1972–1974 concerning the optics of ultracold neutrons (UCN) are briefly reviewed and compared with later results obtained in this field. The basic stages of the development of a neutron microscope based on UCN are briefly described. The possibility of using neutron-optical methods for fundamental investigations with long-wavelength neutrons is discussed.