  We give a solution of the wave equation for massless, or massive spin-2
particles propagating in a gravitational background. The solution is covariant,
gauge-invariant and exact to first order in the background gravitational field.
The background contribution is confined to a phase factor from which
geometrical and physical optics can be derived. The phase also describes
Mashhoon's spin-rotation coupling and, in general, the spin-gravity
interaction.
