An extremely thin cell (ETC) was used to analyse ground state effects of Cs atoms excited by laser light and placed in a magnetic field that was scanned around zero. The advantage of using the ETC is the possibility to separate the hf transitions, which are overlapped in ordinary cells, as a result of the spatial anisotropy of the atom-light interaction. Cs atoms were irradiated by circularly/linearly polarized light at lambda~852 nm, and the dependence of absorption on the external magnetic field was measured.