When light passes into a medium, which angle is referred to as the 'angle of refraction'?

The angle of refraction is defined as the angle made after light has passed through the interface of two different media and has entered the second medium. When light travels from one medium to another, such as from air to water, it changes speed and direction at the boundary between the two media.

According to Snell's Law, this change in direction is quantified by the angle of refraction, which is measured relative to the normal line (an imaginary line that is perpendicular to the surface at the point of incidence) in the second medium. This is why the angle referred to in the question specifically pertains to the angle made after light transitions into the new medium, distinguishing it from other angles related to the incident light's path or absorption characteristics.

Understanding the angle of refraction is crucial when analyzing optical phenomena, such as lenses and prisms, where light behavior as it travels through different materials is essential for predicting outcomes in optics.