One exhibit showing the automatic opening of a door, controlled by a photoelectric cell.

This automatic door relies on a photoelectric cell, or photocell, to function. Photocells are electrical components that produce a current in response to light.

There is a beam of visible light falling on the photocell which is interrupted when a person passes through it. This interruption triggers the photocell to send a signal to the door opening mechanism or 'thruster': a hydraulic jack that pushes the door open.

This type of automatic door was invented in 1931 by American engineers Horace H. Raymon and Sheldon S. Roby. This door was originally displayed in the Science Museum as part of an exhibition on photoelectric cells. It was installed in the basement in 1933 and has been in almost constant use since then.