Automatic doors open as you go near them and shut behind you when you go away from them.Automatic doors are complex machines which have several parts which work together to produce this effect.like a sensor and motor system.
The sensor that is attached on the top of the door sends out a special light that identify the presence of a person near the door.The beam of light is interrupted by that object standing at the door.This causes a disruption in the ''close door'' signal to a sensor to be interrupted.This interruption causes the door to open.once the object has moved out of sensor range and the signal is again transmitted and the doors close.