What happens to light as it passes through a polaroid

In this simple model we represent the polariser by a thin slit. The incoming light has its electric field vectors scattered all over the plane perpendicular to the direction of propagation. The vectors which are parallel to the slit get through unscathed. For the remainder, as illustrated in Figure 8.21, only the component parallel to the slit emerges. These vibrations emerge polarised in the direction of the slit and reduced in size.

Changing the plane of polarisation

Let us consider a light wave which is already polarised when it enters a polaroid. The fraction of the intensity transmitted will depend on the relative direction of polarisation. A law named after Etienne-Louis Malus (1775-1812) follows directly from Figure 8.22. If the electric vector of the incident light has an amplitude E0 and is at an angle d to the slit, only the component in the direction of the slit (E0cos d) will be allowed through.

The intensity of the transmitted wave is proportional to the square of the amplitude:

Intensity of light transmitted I — cos e (Malus's law)

E0 cos 9 = transmitted wave amplitude Figure 8.22 Malus's law.

'Crossed' polaroids

Let us next consider two polaroids arranged so that their axes of polarisation are perpendicular. The light transmitted by the first polaroid is polarised at an angle, 9 = 90°, relative to the second slit and no light will get through.

2nd polaroid (horizontal)

2nd polaroid (horizontal)

Polarising materials

Transparent materials that transform unpolarised light to plane polarised light have a characteristic molecular structure.

Typical polarising material: The molecules tend to be aligned in one direction and look somewhat 'stretched'.

Typical non-polarising material: The atomic arrangement is too symmetrical to let the light squeeze through with its electrical energy absorbed in all but one single direction.

Polarisation and spiders

The Drassodes cupreus spider has a pair of optical sensors behind its principal eyes. These are not 'normal' eyes, but filters which sense the direction of polarisation of sunlight, which varies as the sun changes position. Research indicates that the 'spider-compass' works best at dawn and dusk and when the spider is out and about searching for food.

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Responses

  • D
    What happen when light passes through polaroids?
    2 years ago
  • Senay
    What is amplitude of polarised light when it is passed through first Polaroid?
    2 years ago
  • linnie
    What happen to light passing through a Polaroid plate?
    5 months ago

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