What defines the angle formed by a ray and the normal line at the point of incidence?

The angle formed by a ray and the normal line at the point of incidence is defined as the angle of incidence. This angle is crucial in understanding how light interacts with different mediums, as it describes the angle at which a light ray strikes a surface compared to an imaginary line called the normal, which is perpendicular to that surface at the point of contact.

When a ray of light strikes a surface, the angle of incidence is measured from the incoming ray to the normal line. This is significant because it directly correlates to the other angles that arise during reflection and refraction. For instance, the angle of reflection, occurring after a ray bounces off a surface, is equal to this angle of incidence, in accordance with the law of reflection. Similarly, when light passes from one medium to another, the angle of refraction is influenced by the angle of incidence according to Snell’s law.

The critical angle refers to a specific scenario in which light is completely reflected within a medium, and it only applies when light travels from a denser to a less dense medium. Therefore, the angle that specifically defines the relationship at the point where light first intersects with a surface is indeed the angle of incidence.