What is the term for the angle between a ray of light that strikes a surface and the normal to that surface?

The angle formed between a ray of light that strikes a surface and an imaginary line perpendicular to that surface, known as the normal, is referred to as the angle of incidence. This angle plays a crucial role in optics as it helps to determine how light behaves when it encounters different media.

When light strikes a surface, its behavior—whether it reflects, refracts, or transmits—is influenced by the angle of incidence. Understanding this concept is essential for various applications in optics, including the design of lenses and understanding visual phenomena. In contrast, the angle of refraction refers to the angle formed when light changes direction as it passes from one medium to another, while the critical angle is the specific angle of incidence that results in total internal reflection for light traveling from a more dense medium to a less dense medium. The refractive index is a numerical value that describes how light propagates through a material but does not pertain directly to angles. Thus, recognizing the definition and importance of the angle of incidence is vital for anyone studying optics.