Media that are transparent to light have a property called optical density. It can be said that the higher the optical density of the medium, the "harder" it is for light to move through it, i.e. the lower the speed of light moving there.

When propagating from one medium with optical density to another, the direction of light propagation changes. Such a phenomenon is called refraction of light. When light is refracted in an optically denser medium, the angle between the cross line of the surface and the ray is always smaller than in an optically sparser medium.

When light is refracted from an optically denser medium to an optically rarer medium, at certain angles greater than the angle of incidence, the angle of refraction should be greater than 90 degrees. At such angles, the refraction of the light beam can no longer take place, instead the light is completely reflected - the light is completely reflected back from the interface of the two transparent media.

If light refracts from an object on its way to the eye, it seems to us that the object is somewhere else compared to its actual location. ​​​​​​​The eye can only estimate the direction of the light that has reached it and the distance of the objects, but it cannot tell whether the light has changed its direction in the meantime or not. Instead, we see the image of the object in the direction of the extensions of the rays entering the eye (viewpoint).