Research has enabled virtual reality (VR) users to interact with the physical world by blending the physical world view into the virtual environment. However, current solutions are designed for specific use cases and hence are not capable of covering users' varying needs for accessing information about the physical world. This work presents RealityLens, a user interface that allows users to peep into the physical world in VR with the reality lenses they deployed for their needs. For this purpose, we first conducted a preliminary study with experienced VR users to identify users' needs for interacting with the physical world, which led to a set of features for customizing the scale, placement, and activation method of a reality lens. We evaluated the design in a user study (n=12) and collected the feedback of participants engaged in two VR applications while encountering a range of interventions from the physical world. The results show that users' VR presence tends to be better preserved when interacting with the physical world with the support of the RealityLens interface.