The angle between the wavefronts and the wave rays is called the angle of incidence. The angle of incidence is measured between the wavefronts and the normal to the surface at the point where the wave strikes the surface. The normal is the line that is perpendicular to the surface at the point of incidence.
The angle of incidence determines the direction of the wave after it strikes the surface. The wave will be reflected from the surface at an angle that is equal to the angle of incidence. The wave will also be refracted through the surface at an angle that is determined by the index of refraction of the material on the other side of the surface.
The index of refraction is a measure of how much the speed of light changes when it passes from one material to another. The index of refraction of a material is greater than one if the speed of light is slower in that material than it is in vacuum. The index of refraction of a material is less than one if the speed of light is faster in that material than it is in vacuum.
The angle of refraction is calculated using Snell's Law. Snell's Law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the index of refraction of the material on the incident side of the surface to the index of refraction of the material on the transmitted side of the surface.
For example, if a wave strikes a surface at an angle of 45 degrees and the index of refraction of the material on the incident side of the surface is 1.5 and the index of refraction of the material on the transmitted side of the surface is 1.0, then the angle of refraction will be 30 degrees.
This means that the wave will be refracted through the surface at an angle of 30 degrees to the normal. The wave will also be reflected from the surface at an angle of 45 degrees to the normal.
