A light gate usually consists of an optical transmitter and receiver mounted in a frame with a gap between the two. Essentially it is a switch that reacts when something passes between the transmitter and receiver, breaking the light beam.
The transmitter is usually a tiny infrared diode which emits an invisible beam of light which is then detected by the receiver. When used in conjunction with a datalogger they can measure the time a moving object takes to pass between them, allowing speed to be determined.
If two sets of light gates are used, the acceleration of a moving object can be measured by calculating the difference in time taken to move between the two sets.
If used with a datalogger it is important to make sure they are compatible. Usually the manual that comes with the datalogger specifies the types of light gates that can be used with it. If in doubt, post your question on our forum or ask your supplier.
Stand alone units are available which essentially act as a switch. They often can be set to 'make' or 'break' the switch when the beam is broken. (open or close the switch) enabling speed and acceleration to be manually calculated when a datalogger is not available.
Light gates are used because of their accuracy. They can be set to react very quickly when the beam is broken which eliminates human reaction time errors.