Three-dimensional goat

Fermat's law is the principle of the minimal (shortest) path. According to the Fermat's principle the optical path L (i.e. the product of a geometrical path in a given medium s and of a total internal reflection index in this medium) taken by light between two points n) is the shortest.

Fermat's principle is the basis on which other properties of light - reflection and refraction - are described. Refraction of light through glass is lesser compared to refraction of light through air: light 'chooses' the shorter path through glass and slightly longer path through air. According to Snelius's 2nd Law the ratio of the sine of the angle of incidence a to the sine of the angle of refraction b is equal to the ratio of velocity a light waves (i.e a constant for a given pair of media).

The three-dimensional image of a goat is created in a cuboid structure with planar surfaces: light beams reflect off three mirrors and are refracted on them. What our eyes perceive, is a kind of the goat's projection onto the planes of the structure. The light beams we can see do not strike the surfaces at a right angle. That is why the image we view is not exactly three-dimensional projection of the goat, it is rather kind of 'axonometry' projection in which we view the goat from three not exactly right-angled directions.