## 2 The State of the Art
### What Do We Understand by AR and VR?  
A way to define AR and VR consists of placing both technologies on the reality-virtuality continuum [32]. In this case, AR would lie closer to the real context, whereas VR would be located at the other end, with \"Mixed Reality\" being situated halfway. It is this digital technology, which incorporates both AR and VR features, that we are going to use in this study.  
These techniques, linked to the possibility of creating new non-physical scenarios (totally or partially) offer computerized simulations, but require physical devices. For ex-ample, immersive VR can be perceived by using devices such as data gloves, special clothing, or a special pair of glasses [33].  
Generally speaking, VR applications in education can be classified, according to visualization and interaction devices [34], into two broad categories: (i) non-immersive (the known window in the world), where the user's worldview is channeled through the flat screen of a computer which serves as a \"window\"; and (ii) immersive, which fully introduces the user to a virtual world through the use of lenses with two small screens placed before that user's eyes. Concerning AR, [PERSON] and [PERSON] [35] point out that it is a technology that permits the combination of digital and physical information in real time by means of various technological devices, such as tablets or smartphones to quote but two, thus creating a new enriched reality.  
Regarding VR, this is the most advanced way to connect a person with a computer system. This connection allows for a direct interaction between the user and the artificially generated environment. The latter serves to stimulate one or all of the human senses and is mainly characterized by creating in the user an illusion of direct participation at the brain level. In turn, [PERSON] [35] clarifies that VR is the technology that makes it possible for any user to immerse themselves, firsthand and in 360\({}^{\circ}\), in virtual scenarios by means of a VR viewer. This makes it easier for users to immerse themselves in such scenarios, even enabling them to interact with the elements which they are made up of. Users stop seeing the place where they find themselves physically and start visualizing \"another reality\" and interacting with it. As can be seen, AR and VR put forward ways to interact with reality in various ways: whereas VR takes users to an unreal world so that they can feel immersed, AR adds information to the real world.  
On another note, it deserves to be highlighted that we can distinguish different levels within AR based on diverse elements such as: a black-and-white artificial pattern (N1); an image (N2); a 3D entity (N3); a location on the planet determined by its GPS coordinates (N4); and a thermal footprint (N5) [36]. According to [PERSON], [PERSON], and [PERSON] [37], these levels do not exist in VR, as they are replaced by various virtual reality systems: desktop or non-immersive; semi-immersive; and totally immersive. In this regard, some authors reduce them to only two types [35]: immersive (based on the simulation of 2D or 3D scenarios where a user perceives such scenarios with a feeling of \"being present\" in the first person, as though they were really there); and non-immersive (based on the visualization of virtual scenarios through a screen, giving users the impression that they are looking at that virtual scenario from a \"window\").