Image calculations are simple when the ray picture is used. In this picture the light travels in straight paths as long as the index of refraction is constant. At the interface between two transparent dielectric media with different indices of refraction the rays change direction according to Snell's law of refraction, n_1 & = & n_2 . Here and are the indices of refraction, and and are the incident- and refracted angle all shown in snell. The point at the interface between the two media where the incident ray hits, together with the incident ray and the refracted ray, span a plane called the plane of incident.
Not all the light will be refracted at an interface, some of it will be reflected. This is described by the equations of Fresnel. The amount of reflected versus refracted light depends on the incident angle, the polarization and the indices of refraction. It can also easily be shown that the reflection angle equals the incident angle. When the incident angle is small most of the light is transmitted. However, there will always be losses, therefore the number of interfaces should be minimized in all optical devices.