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Object tracking is a main application of wireless sensor networks (WSNs), and has been studied widely. In this work, we study multiple objects tracking problem using WSNs, in which we assume no equipment is carried by the object and the tracking procedure is passive. We first show that without carefully design, Received Signal Strength Indicator (RSSI) and Link Quality Indicator (LQI) are not as effective as claimed for passive detection through our testbed studies. We further propose to use light to track moving objects in WSNs. To our best knowledge, this is the first work to study multiple objects tracking using light sensors and general light sources. We propose a novel probabilistic tracking protocol to track multiple objects. We further design several efficient methods to compute some attributes of the moving objects (like heights, moving speeds etc.).