Light gate timer experiment. This equipment is used to measure the velocity of a trolley moving on a rail. As the trolley passes the light gate, it breaks the light beam passing from the source to a photocell (lower left). The timed interval is relayed to a laptop computer (background) that is displaying the information in an on-screen graph. Combining this information with readings from another light gate a known distance away, or using a known length of card (blue) on the trolley, allows the speed to be calculated.