When transmitted from one medium with a refractive index of n1 into a denser medium with a refractive index of n2, the rays of light are diffracted towards the axis of incidence (ε1> ε2). For the transition from air to glass the refractive index is approx. n2/ n1=1.5.

When transmitted through a medium of a different density, rays are displaced in parallel.

When beams of light enter a clear transmitting medium of differing density, e.g. from air into glass and vice versa from glass into the air, they are refracted, i.e. the direction of their path is changed. In the case of objects with parallel surfaces there is only a parallel light shift, whereas prisms and lenses give rise to optical effects ranging from change of radiation angle to the concentration or diffusion of light to the creation of optical images. In the construction of luminaires refracting elements such as prisms or lenses are frequently used in combination with reflectors to control the light.