The Cathode Ray Tube
At the 'heart' of the oscilloscope is the cathode ray tube.
One end of the evacuated tube is coated with fluorescent material which glows when hit by electrons. At the other end, a heated cathode emits a cloud of electrons which is focused and accelerated as a narrow beam towards the fluorescent coating.
On its way, the electron beam passes between two pairs of metal plates. When a voltage is applied to these plates, the electron beam is deflected from its straight path.