The study of real numbers with an emphasis on the High School students is underexplored in research in Mathematics Education in Brazil and other countries. In the literature review, we note that rational and irrational numbers are not part of the repertoire of knowledge of most prospective teachers, despite their school experience as students, for several years. This is a rich field of research because of its relevance in the Mathematics curriculum in High School. This thesis examined the conceptual images of students after they had been encouraged to write and to talk about real numbers, driven by activities in which the use of a calculator was stimulated. We conducted an analysis of documents of this subject. We elaborated and carried out activities, prioritizing the ideas of density and infinity. We used finite and infinite decimal representations and irrational square roots, and chose the division algorithm as a trigger of this process. The clinical-qualitative methodology allowed us to collect data in two stages: the preliminary exploratory study, by applying a questionnaire to 70 students; and the main study, conducted with 12 students, in which the students were constantly asked to explain their judgments. The research environment has enabled the demonstration and the analysis of prototypical and analytical judgments that were identified by relevant and irrelevant attributes. The results indicate the need for an effective work, from the division algorithm, which consolidates the knowledge about decimal representations. Increasing the repertoire of positive and negative examples related to the notions worked seems to stimulate conceptual image descriptors that are favorable for the development of the concept of real number.