Today we are going to be talking about polarisation.

Now what does it mean for a wave to be plane polarised?

A wave is plane polarised if the particles or the electromagnetic field oscillate in a single plane only.

And this is a property of transverse waves.

So both of these aspects are very important for this definition.

Let me illustrate this.

For instance, you could have a piece of string.

That piece of string could be oscillated in multiple different directions.

For instance, we could choose to oscillate this in the vertical plane, like so.

So each of those particles are going to be moving up or down along here.

So each of them is going to be going up or down, up or down, up or down.

However, we can also choose to oscillate this piece of string in the horizontal plane as well.

In that case, the particles are going to be moving horizontally along this plane, like so.

And a wave is said to be plane polarised if the particles oscillate in a single plane.

Let's have a look at a few conventions for denoting some different types of polarisation.

Let's start off with some unpolarised waves.

For instance, sunlight tends to be unpolarised.

So this means that the electromagnetic field is oscillating in many different planes.

Some of it is going to be in the horizontal plane, some of it is going to be in the vertical plane.

And this is denoted with the following symbol.

So essentially it's a cross with denoting that the particles are moving both up and down.

They have a component moving up and down, a vertical component.

And they also have a horizontal component, which denotes that the particles or the electromagnetic field will be moving horizontally as well, or a mixture of all of them.

If the particles or the EM waves are polarised in the vertical plane, we often denote this simply with an arrow moving up and down, like so.

And if it's horizontally plane polarised, we can also denote this with a horizontal arrow to show that the electromagnetic field or the particles are oscillating in the horizontal direction.

OK, guys, well, let's have a look at how this works in terms of polarising filters.

Let's imagine that we have some unpolarised light, for instance, and we can see that the EM wave is oscillating in both planes in this case.

As soon as it reaches what is known as a polarising filter, the component which is oscillating in the horizontal direction will be absorbed and only light which is oscillating in the vertical plane will pass through the polarising filter.

So this means that the light will look like this as it emerges out of the polariser.

OK, guys, now let's add a second polarising filter.

And let's make sure that we rotate the second polarising filter at 90 degrees to the first one.

So the first one, you can see it had a vertical orientation and this one has a horizontal orientation.

In this case, light which is vertically polarised will be absorbed.

So this means that no light is going to go through as there's no component oscillating in the horizontal plane and the second filter will only let light through which is oscillating in the horizontal plane.

OK, guys, so hopefully you've enjoyed my explanation of polarisation.

Thank you very much for watching and I'm going to see you in the next video.