20.28 Polarised light

Before you read this, I suggest you read post 19.9.

fig1

In post 19.9, we saw that light can be considered as an electromagnetic wave. This wave consists of an electric field and a magnetic field that oscillate perpendicular to the direction of propagation of the wave, as shown in the picture above. The electric field can be represented by an electric vector and the magnetic field by a magnetic vector. We can consider that the picture shows the wave at an instant in time; then the direction of propagation represents distance along the wave. Alternatively, we can consider that the picture shows the wave at a fixed point in space, then the picture shows how the electric and magnetic vectors depend on time (see post 18.10). Then the picture is a graph of the electric and magnetic fields plotted against time, where the time axis of the graph is the direction of propagation of the wave.

If the electric and magnetic fields continue to oscillate in the same directions, the wave is plane polarised (post 19.9).

How can we produce plane polarised light? The simplest method is to pass unpolarised light through a sheet of material called Polaroid. Polaroid consists of parallel polymer (post 20.7) molecules. The molecules are aligned parallel by stretching Polaroid in the manufacturing process – in the same way that rubber molecules are oriented by stretching (post 20.11). If the electric vector is in the same direction as the oriented molecules, the oscillating electric field will exert an oscillating force on the charged electrons in each polymer molecule, causing them to oscillate to oscillate (post 17.24). Since electrons have charge (post 16.27) the oscillating electrons act as a source of electromagnetic waves (post 19.9). So the light passes through the polaroid. If the electrons can oscillate only in the direction of the oriented polymer chains, the light that is transmitted will be plane polarised with the electric vector in the direction of the oriented polymers – this is what happens when light passes through polaroid. The plane in which the electric vector oscillates is called the plane of polarisation. In this post, I shall call the direction of the oriented polymers the direction of polarisation.

fig2

In the picture above, a beam of light is perpendicular to a sheet of Polaroid. The electric vector, E, makes an angle, α, with respect to the direction of polarisation. Then the electric vector of the light that is transmitted by the Polaroid is plane polarised in the direction of polarisation with a magnitude Ecosα that is the component of the electric vector in this direction (see post 16.50).

When α = 0, cosα = 1 and the electric field and, hence, the intensity (post 19.9) of the light is unchanged. When α = 90o, cosα = 0 and so the electric field is zero and no light is transmitted.

fig3

Now let’s consider putting a second sheet of Polaroid, after the first, with its direction of polarisation perpendicular to that of the first – as shown in the picture above. The electric vector, of the light incident on the second sheet, is perpendicular to its direction of polarisation – so no light passes through. The two sheets of Polaroid with their directions of polarisation perpendicular are called crossed polars and lead to extinction of the light incident on the system.

Let’s suppose that something rotates the direction of the electric vector, about the direction of the light beam, between Polaroids 1 and 2. In order to achieve extinction, we would need to rotate polaroid 2 by the same angle, about the same axis. The rotation applied to the Polaroid could then be used to measure the rotation of the electric vector. A device that does this is called a polarimeter – Polaroid 1 is then called the polariser and Polaroid 2 the analyser.

This post provides information that we will need to understand more about optical isomers (post 20.27).

 

Related posts

19.21 Refraction
19.20 Diffraction
19.9 Electromagnetic waves

Follow-up posts

20.29 Properties of optical isomers

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