In terms of optics, what does 'normal' refer to?

In optics, the term 'normal' refers to a line that is drawn perpendicular to the surface at the point where a ray of light strikes that surface. This concept is crucial for understanding how light behaves when it encounters different mediums, such as when it refracts or reflects. When analyzing the path of light, the normal line serves as a reference point, helping to measure angles of incidence and refraction according to laws such as Snell's law. Therefore, recognizing the importance of the normal line helps to accurately predict how light will move through and interact with various materials.