Abstract: A mobile device is provided, which includes a camera unit, a sensor unit, a see-through display, and a processor. The camera unit takes an image of a finger and a surface. The sensor unit generates a sensor signal in response to a motion of the finger. The taking of the image and the generation of the sensor signal are synchronous. The see-through display displays a GUI on the surface. The processor is coupled to the camera unit, the sensor unit, and the see-through display. The processor uses both of the image and the sensor signal to detect a touch of the finger on the surface. The processor adjusts the GUI in response to the touch.