angle of incidence symbol: θ

of electromagnetic radiation
https://doi.org/10.1351/goldbook.08222
Angle between a beam of electromagnetic radiation and a line normal to a surface.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 651 [Terms] [Paper]