This study aims in the comparison of teaching of the Physics via experiment and the traditional blackboard teaching and investigates the effect of each method in the students? performance both in the short and the long run. It was applied in Class B students of Gymnasium and the subject of the lesson was ?Reflection and Diffraction of Light?. The target group for our research was 372 students from different size of towns from the prefectures of Ioannina and Arta. 202 of them were taught experimentally conducting experiments themselves, while the remaining 170 were taught traditionally. With the use of questionnaire we examined the following main objective: Does the teaching via experiment yields better results from the traditional teaching, and is the study subject impressed better in the students in the first case? We gave out questionnaires to the students during the following course as well as two months later. Also for better, follow-up control we handed out to two representative classes the same questionnaires six months lather. The research results indicated that the concepts taught via experimental activities were better impressed to the students. It is remarkable that knowledge obtained by experimental teaching, is not only assimilated by the students, but most importantly, it is maintained for a satisfactory interval of time.