Audio and Video tapes
Audio and video tapes make use of magnetic materials to record sound and pictures. These tapes have a thin layer of iron oxide on the surface.
In the simple case, an voice recorder consists of a tiny electromagnet next to the tape. The sound wave is converted by electronics into oscillating magnetic field near the electromagnet. This field magnetises a tiny area on the tape.
As the sound wave oscillates in direction and strength, the magnetic field also changes in direction and strength in the same way.
As the field changes, the tape is pulled along by a small motor in the recorder. Different positions along the tape get magnetised in different directions and strengths, following exactly the same pattern as the sound wave.
During playback, the tape is pulled along, staying close to the electromagnet. The electromagnet does not receive any current from the coil this time. Instead the changing magnetic field induces an electric current in the coil, according to Faraday's law of electromagnetic induction.
This tiny electric current follows exactly the same pattern as the variation in magnetic field on the tape, which in turn has the same pattern as the sound that was recorded.
This tiny current is amplified by electronics and sent to a loudspeaker. We then hear the recorded sound.
Video tapes are more complicated. The information of colour and brigtness at every point on the picture has to be converted into a form that can be recorded on the tape. But the way that each bit of information is recorded is the same.