Geodesic
The edge rays inside a parabolic screen
Zapiski Nauchnykh Seminarov POMI, Mathematical problems in the theory of wave propagation. Part 12, Tome 117 (1981), pp. 112-122 
A. Ya. Kazakov; O. V. Graunov. The edge rays inside a parabolic screen. Zapiski Nauchnykh Seminarov POMI, Mathematical problems in the theory of wave propagation. Part 12, Tome 117 (1981), pp. 112-122. http://geodesic.mathdoc.fr/item/ZNSL_1981_117_a6/
@article{ZNSL_1981_117_a6,
     author = {A. Ya. Kazakov and O. V. Graunov},
     title = {The edge rays inside a~parabolic screen},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {112--122},
     year = {1981},
     volume = {117},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1981_117_a6/}
}
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%A O. V. Graunov
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Voir la notice du chapitre de livre provenant de la source Math-Net.Ru

The study deals with the rays that come out of an edge of a parabolic screen and after reflections by the concave side of the screen go away from the screen at some fixed direction. The formulas for calculation of a path and disconvergence of such rays are given. In the case of parabolic screens a limiting direction occurs. This direction proved to be caustic bound.