Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to establish a time standard (such as an atomic fountain), as a way to measure velocity or path length, or as a way to learn about the particle or medium's properties (such as composition or flow rate). The traveling object may be detected directly (e.g., via an ion detector in mass spectrometry) or indirectly (e.g., by light scattered from an object in laser doppler velocimetry).