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# Polarisation - what is it and what is it used for?

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Introduction

Jodie Tarbin

Polarisation – what is it and what is it used for?

Natural sunlight (and most other forms of illumination) transmits light waves whose electric field vectors vibrate in all different planes relating to the direction of transmission.  When these electric fields are restricted to a single plane by filtration, the light is said to be polarised as all of the light waves are vibrating in the same plane.  When unpolarised light is passed through a Polaroid filter, it emerges with only half the intensity of before, and with all waves travelling in the same plane.  This light is now polarised.

As unpolarised light strikes the filter, the quantity of waves vibrating in a certain direction are absorbed by the filter.  By eye it cannot be seen which direction has been absorbed, but the general rule is that ‘the electromagnetic vibrations which are in a direction parallel to the alignment of the molecules within the Polaroid are absorbed.’

Middle

Mirrors are not good polarisers, although many transparent materials are much

better, as long as the incident ray is within a certain angle limit.  The angle inducing maximum polarisation is known as Brewster’s angle and is given by the expression:

Where n is the refractive index of the medium, Ө (i) is the angle of incidence, and Ө(r) is the angle of refraction. This type of reflected light is often called glare and can be blocked by a pair of polarised sunglasses.

The lenses of the sunglasses have polarising filters that are aligned vertically with respect to the frames.  In figure 4 (below) the blue light waves have their electric field vectors orientated in the same direction as the polarisation axis and are therefore passed through the filters in the glasses.  Nevertheless, the red wave is perpendicular to the polarisation axis and is therefore blocked by the lenses to protect the eye and allow glare to be reduced.

Conclusion

0 twist of the light so it can then pass through polariser two, which is polarised horizontally and is perpendicular to polariser 1.  The light forms one of even segments on the display, as shown in the diagram.

Polarisation can actually prove that light waves are transverse.  If you take a vertically polarised wave and put an object with a vertical slit in its path, it will still go through.  However, if you rotate the object so tat the slit is now horizontal, the vertically polarised wave will be blocked and won’t get through the filter.  This is true for transverse waves, nevertheless as longitudinal waves ‘only vibrate along the direction of motion’, neither orientation of the slit would stop them…therefore longitudinal waves cannot be polarised! This proves that light is transverse as we know that light can be polarised.

Overall polarisation is useful for a lot of technology and it helps us a lot during day to day life.

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