We analyse the behaviour of a focus wave mode incident on a dielectric plane. The Descartes-Snell law is valid for the reflected beam but the refracted beam undergoes a Goos-Hänken shift and propagates in the direction of the incident focus wave mode. The Fresnel coefficients do not depend on the angle of incidence. Onde-flèche sur un interface diélectriqueRésumé. On analyse le comportement d'une onde-flèche incidente sur un diélectric plan. La loi de Descartes-Snell est valide pour le faisceau réfléchi mais le faisceau diffracté subit un déplacement de Goos-Hänken et poursuit sa propagation dans la direction de la flèche incidente. Les coefficients de Fresnel ne dépendent pas de l'angle d'incidence.

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Nonlinear periodic drift waves are investigatcd on the basis of a simple perturbation scheme for both the amplitude and inverse frequency. The coefficients for the generation of the forced harmonics are derived, a nonlinear dispersion relation is suggested and a criterion for the onset of the modulational instability is obtained. The results are compared with those obtained with the help ofa standard KBM ireaimeni. Moreover, cnoidai driii waves are suggested and compared !O an experimentai observation.