In this article we use phase resolved optical emission spectroscopy to study emission pattern in plasma sheath of dual frequency capacitively coupled plasma in Ar and Ar-O2 discharge. Two emission patterns are found in sheath region of radio-frequency coupled powered electrode. The first pattern is related to electron impact excitation because of the sheath expansion. The second pattern is caused by electron impact excitation of secondary electrons. Two emission patterns are also highly modulated with the l...