Can we watch the goat from all three different sides without walking around it? As you can see on the photo - yes! But the goat has to be placed in a glass cage. The same with the bear: first he covers his ear and than he uncovers it.

The light beam choose a strange path; they take a shortcut. Instead of taking the straight path to our eye, they first go at an angle, take a straight path to the wall of the cage, they refract on the glass wall and then they go along another straight path to our eye. Instead of choosing one line, they choose a chained path.

It turns out that in reality this route is shorter, but not on paper, but in time. We call it Fermat's principle and it is the basis for the refraction of light (so called Snelius's Law).

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.

L = ns

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 α to the sine of the angle of refraction β ) 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.