Disclosed are methods and devices for rendering interactions between virtual and physical objects on a substantially transparent display are disclosed. In one embodiment, the method includes displaying a user-interface on a substantially transparent display of a wearable computing device. The method further includes displaying a virtual object in the view region at a focal length along a first line of sight and detecting a physical object at a physical distance along a second line of sight. The method still further includes determining that a relationship between the focal length and the physical distance is such that the virtual object and the physical object appear substantially co-located in a user-view through the view region and, responsive to the determination, initiating a collision action between the virtual object and the physical object.