Incidence of a beam of
electromagnetic radiation when the
angle of incidence \(\theta = \pu{0\!^{\circ}}\), i.e., the incident radiation beam is normal to the surface.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 756 (https://doi.org/10.1515/pac-2019-0203)