Light bouncing off objects allows us to see them.

Light is an oscillating electromagnetic wave.

We see different frequencies of that oscillation as different colours.

The electric field component of a light wave can oscillate in any direction perpendicular to the direction of travel. Light from the sun oscillates in all directions, we say it is unpolarised.

If the waves have a preferred direction of oscillation, they are polarised.

For example, light from your phone is vertically polarised.

Your polarised sunglasses are a polarised filter.

Here, they let vertical polarisation through.

If we rotate them, almost all the polarised light gets blocked. Light produced by a torch is unpolarised.

When we place a polarised filter in between, the light becomes polarised.

In space, randomly moving hot gas or plasma emits unpolarised light, like the torch.

Plasma threaded by a magnetic field is ordered and the emitted light is polarised.

The magnetic field acts as a polarised filter.The strong gravity near a black hole bends the magnetic field and twists the polarisation direction of the light from the surrounding plasma.

The polarised light rays that manage to escape travel to a distant camera.

The intensity of the light rays and their direction are what we observe with the Event Horizon Telescope.

Using this knowledge, we can map out the magnetic fields that surround black holes.