Question:Describe how infrared radiation is used in automatic doors.
Automatic doors detect an increase in the amount of infrared radiation being detected, which indicates a person approaching because people are hotter than the surrounding environment.
There are other systems for automatic doors. Infrared would not recognize a person if the temperature of the air was 37oC, which happens in some countries regularly.
Note: That is is not the case that the surrounding environment (the air or the room etc) does not emit infrared radiation; it is just that warmer objects, such as human bodies, emit more.
Note: It is not the case that the doors detect movement.