Author(s):
1. Ivana Radonjić Mitić, University of Niš, Faculty of Sciences and Mathematics, Department of Physics, Višegradska 33, Niš, Serbia
2. Iliya Zhelezarov, Technical Univerity of Gabrovo, Hadji Dimitar Str. 4, 5300 Gabrovo, Bulgaria, Bulgaria
3. Plamen Tsankov, Technical University of Gabrovo, Department of Physics, Chemistry and Ecology 4 Hadzhy Dimitar str., Bulgaria

Abstract