https://doi.org/10.1351/goldbook.08647
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.
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.