An amorphous material is a solid whose molecules are not arranged in a crystal lattice but are randomly distributed. Basically, the molecules are disordered as in the liquid state, but in immobile form. In general, an amorphous substance enters the solid state when the molecules lose their free mobility due to cooling and at some point are no longer able to move in their position. Amorphous substances are often those consisting of large macromolecules which already have a high viscosity in the melt and become so immobile when cooled down that they can no longer arrange themselves. This is particularly true for plastics, which are often also semi-crystalline because they form a kind of crystal lattice in some areas. Further amorphous materials are those to which foreign molecules are added which disturb the formation of the crystal lattice of the basic material. The amorphous structure of glass is based on this principle, for example. The absorption of light is attributable to the resonance excitation of the crystal lattice, which is why crystal lattices are opaque. Since no absorption takes place without a crystal lattice, most amorphous substances are transparent. Non-transparent amorphous substances usually contain crystalline areas, for example admixtures of a crystalline material. Usually, the cooling-down speed of the melt can be used to determine how crystalline or amorphous the solid matter will be. When cooled down very quickly, the molecules do not have enough time to arrange themselves in the crystal lattice before solidifying – they remain amorphous.