Neutron reflectometer with horizontal scattering geometry
Neutrons are no longer reflected from a single surface but from two semi-transparent surfaces. The reflectivity curve still has a total reflection plateau but the descending portion shows the presence of oscillations resulting from interference between beams reflected by the two surfaces. The size of these oscillations is related to the layer thickness.
The actual situation is generally much more complex (multi-layers, rough or diffuse surfaces, etc.) and interpretation of observations is realised by model-based computer simulations.
Note that this animation shows a reflectometer operating in the monochromatic mode while D17 is more often used in the time-of-flight mode.