You face each other, the lens in the middle, so that you can look at your partner like through a magnifying glass. If you look through the Fresnel lens alone, you can see the space behind the lens upside down.
Further informationson the exhibit "Fresnel-Lens"
The Fresnel lens, invented around 1822 by the French physicist Augustin Jean Fresnel, hardly differs in its image from an ordinary collecting lens such as a magnifying glass: magnification in the vicinity and upside-down image in the distance.
The lens, however, follows a construction principle that generally makes the weight and volume of large lenses smaller. The volume of the Fresnel lens is reduced by dividing it into ring-shaped areas. In each of these areas, the thickness is reduced, giving the lens a series of annular steps. Since light is refracted only as it passes through the lens surfaces, the angle of refraction does not depend on the thickness but only on the angle between the two surfaces. The lens retains its properties, only the image quality is degraded by the step structure.
Fresnel lenses are used in lighthouses, headlights, projectors or also as a lens in rear windows of camping buses, for example, in order to reduce the "blind spot".