## 3 Design Considerations of a VRLE  
Prior studies [1] have reported that the development of a VRLE should be carried out following a design process (Figure 3) that involves steps to: (i) decide the level of realism required to achieve the objectives of the VRLE; (ii) choose the level of interactivity for the VRLE; (iii) select the software and hardware that best suits the development needs arisen from the previous steps; (iv) simulate the virtual environment and program the interactivity; and (v) test the application with pilot users and make the required modifications upon analysis of the results from such tests. Nonetheless, based on the authors' own experience, the design process shown in Figure 3 does not guarantee the achievement of meaningful learning. Thus, we recommend in this article to include a robust step-by-step protocol in the design process of those VRLEs that simulate realistic laboratory experiments. In fact, the concept of a step-by-step protocol for a VRLE implies that it meets the following criteria: (i) displays a sufficient level of interactivity to carry out the virtual experiment in a motivating and effective way at the formative level (i.e., if the level of interactivity is low, the user does not interact with the VRLE and does not retain knowledge; however, if the interactivity is too high, the user can lose the thread of the experiment and become unmotivated); (ii) always indicates to the user what is the next step to take and how to complete it; and (iii) does not allow the user to carry out unnecessary actions or lead the user to fail the experiment. The step-by-step protocol of a virtual experiment hence helps the user to focus on understanding each stage of the experiment, avoiding the need to spend a lot of time learning how to use the VRLE [41]. Figure 4 shows the result of adapting the step-by-step protocol to the prior flow chart of Figure 3. \begin{table}
\begin{tabular}{l l c c} \hline \hline  & **Feature** & **VRLEs (5 years ago) Figure 1** & **VRLEs (updated) Figure 2** \\ \hline \(\bullet\) & Light bounces according to optic equations & No & Yes \\ \(\bullet\) & Realistic physically based rendering materials & No & Yes \\ \(\bullet\) & Virtual environment subject to laws of physics & No & Yes \\ \(\bullet\) & Easy adaptation to platforms other than the computers & No & Yes \\ \(\bullet\) & Possibility of adaption to immersive virtual reality (IVR) & No & Yes \\ \(\bullet\) & High knowledge in programming required for & Yes & No \\  & development or updates & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of main characteristics of VRLEs developed by the authors at different stages. Figure 3: Design process of a VRLE previously described and published elsewhere. The main difference between both flow charts (Figures 3 and 4) is the level of attained interactivity which will vary depending on the aim of the VRLE, namely to: (i) help the user to learn how an experiment shall be carried out, or (ii) help the user to understand a concept. When a student conducts an experiment in a real laboratory, before starting the actual test, he/she receives a procedure from the teacher that contains a detailed sequence of steps that the student must follow from beginning to end, for successful completion of the test. Similarly, a student who performs a virtual experiment in VRLEs that simulate real experiments (by means of a step-by-step protocol) should receive the same detailed steps that the student would follow in a real laboratory. By contrast, when the objective of the VRLE is to help the student to understand a basic concept, different levels of interactivity can be chosen for distinct scenarios: (i) a step-by-step approach, which would allow little freedom of action; (ii) an open world, which would allow the student to freely explore with a high degree of freedom. Another difference between Figures 3 and 4 is the amount of information that is displayed to the user while using a VRLE.