  The force from radiation pressure owing to the grating momentum was measured
for a thin transmissive fused silica grating near the Littrow angles at
wavelengths of 808 nm and 447 nm. A significant magnitude of force was measured
in the direction parallel to the grating surface. We also confirmed that the
component of force normal to the grating surface may vanish. This forcing law
is characteristically different from radiation pressure on a reflective
surface, and thus, opens new opportunities for light-driven applications such
as solar or laser driven sailcraft, or the transport of objects in liquids.
