Door Switch Mortice 2A Push to Break
Width - 15mm, Height - 48mm
This switch is designed to be sunk into a door frame. When the door is opened, the switch is released to switch on the light / sound a buzzer / whatever you want it to switch. When the door is closed, the switch is pressed back in, breaking the circuit.
Before starting any electrical work, always switch off at the mains. If in doubt consult a competent electrician