"垂直入射" can be translated as "normal incidence" in English.
In physics and optics, this term is commonly used to describe the situation where a wave (such as light, sound, or an electromagnetic wave) strikes a surface or boundary at a 90-degree angle (perpendicularly).
For example:
"When light is incident normally on a surface, it is called normal incidence."（当光垂直入射到一个表面时，称为垂直入射。）