When light passes through the smooth interface and enters the second medium, it will change direction according to the following rules: the incident ray, the refracted ray, and the interface normals passing through the incident ray and the point of incidence are all in the same plane; as shown in Figure 1, if the incident ray In a medium with a refractive index of nm and the angle with the interface normal is θ1, the refracted ray is in a medium with a refractive index of n2 and the angle with the interface normal is θ2, then formula 1

In the formula, θ1 and θ2 are the angles between the normal and the light on both sides, respectively. Suitable for most common materials such as glass, clear plastics and liquids. For some crystals and stressed transparent solids, the laws of refraction do not apply.

When light enters from a high refractive index medium to a low refractive index medium, such as from glass to air, there will be refracted rays only when the incident angle is less than the critical angle sin-1 (n2/n1) (see Figure 2). When the angle is greater than the critical angle, the light is completely reflected. For example, in glass with a refractive index of 1.5, the critical angle is 1=41°49′. If the incident angle of the light is greater than the critical angle, then there is no refracted light, and all the energy of the incident light is reflected. This phenomenon is called total reflection.

Total reflection provides a method of ideal specular reflection and has been widely used in the manufacture of prismatic binoculars, reflective signals and lamps. Another application is what is known as fiber optics, where light travels along flexible transparent rods or fibers.