What do we call the angle between the incident ray and the perpendicular to the boundary at the point of incidence?

The angle between the incident ray and the perpendicular to the boundary at the point of incidence is referred to as the angle of incidence. This angle is a fundamental concept in optics, particularly in the study of how light behaves at the interface between two different media.

When light travels from one medium to another, the angle of incidence is crucial for understanding how much the light will bend (or refract) as it enters the new medium. The relationship between the angle of incidence and the resulting angle of refraction is described by Snell's Law, which helps us predict the behavior of light at boundaries.

Understanding the angle of incidence is essential for various practical applications, such as designing lenses and optical instruments, as well as in fields like photography and astronomy where light manipulation is key. This concept serves as the foundation for further exploring phenomena such as total internal reflection and the critical angle, which are advanced topics that build upon the basic principle of the angle of incidence.