glancingangle

Glancing angle, also known as grazing angle, is the angle between an incident beam and the surface on which it strikes, measured with respect to the surface plane. It is the complement of the conventional angle of incidence measured from the surface normal. A small glancing angle means the beam meets the surface almost parallel to it, producing a grazing incidence.

In optics and surface science, the glancing angle determines how much of the beam is reflected, scattered, or transmitted. For X-rays, reflectivity is high only at very small glancing angles below a material- and wavelength-dependent critical angle; above that angle, X-rays are largely transmitted or absorbed. This principle underpins grazing-incidence optics, where mirrors are operated at shallow angles to reflect X-rays efficiently.

Applications include grazing-incidence X-ray reflectometry (GIXR) and grazing-incidence small-angle scattering (GISAXS), as well as other surface analysis techniques used to characterize thin films, multilayers, interfaces, and surface roughness. By varying the glancing angle, researchers can infer film thickness, density, and interfacial properties.

Outside optics, the term also appears in discussions of oblique or glancing impacts, where a projectile or wave strikes a surface at a shallow angle, often resulting in reduced penetration or deflection compared with a normal incidence.

Note that different disciplines may define the reference angle differently, so specifying whether the angle is measured from the surface plane or from the surface normal is important for clear communication.