  We solve the problem of beams of phonons and rotons incident on, and
interacting with, solid surfaces. Phonons and rotons are the quasiparticles of
superfluid helium and have a unique dispersion curve. The dispersion curve
controls the transmission, reflection and mode change of these quasiparticles
at the interface with another medium. We develop a non-local hydrodynamic
theory in a consistent and unified way. The structure of the solutions in the
quantum fluid is discussed. The creation probabilities of all quasiparticles
are derived when any one of them is incident on the interface. The dependencies
on frequency and angular are analysed and the backward reflection and
refraction for $R^-$ rotons are discussed.
