Polaroid

According to scientists, light waves are a type of electromagnetic waves. Electric and magnetic fields in light waves are mutually perpendicular to each other and vibrate in planes perpendicular to the direction of propagation of the wave. This happens only if the wave travels in a direction perpendicular to the plane of the paper. If so, then the electric and magnetic vibrations will be in the plane of the paper.

A Polaroid is a large film placed between two glass plates. To make this, microscopic crystals of the organic compound herapathite or iodo sulphate of quinine are spread on a thin sheet of nitro-cellulose by a special method in such a way that the optical asymmetries of all the crystals remain parallel. These crystals are dichromatic. 

Polaroids have two primary uses:

(i) Polaroid filters can be used in glasses to reduce glare.

(ii) In chemistry, polaroids are used to determine the chirality of molecules.

Mechanism

In non-polar light, the electric vectors are in all directions. When a light beam is introduced on a Polaroid film, it splits into two plane-polarised rays. In one beam, the electric vectors are parallel to the axis of the herapathite crystal and perpendicular to the axis in the other. Out of these, the ray with the electric vector perpendicular to the axis of the herapathite is completely absorbed. In this way, the emitted light is entirely polarised. 

The light emitted from a polaroid is plane-polarised.

When the second Polaroid is rotated by 90° and brought to the cross position, light does not transmit through them. In this case, the polarisation directions of both the polaroids are perpendicular to each other. In this case, the polaroids are crossed polaroids. In the above process, the first one is called (analyser). 

When a beam of polarised light passes through a Polaroid film, the film allows only those components whose electro-vectors vibrate parallel to the Polaroid’s direction of polarisation. Thus, the transmitted light is plane-polarised. 

In unpolarised light, the light waves may have vibrational planes. Semi-polarised light can have two perpendicular planes of vibration. Perfectly polarised light can have only one plane of vibration.

An easy way to differentiate between these three types of light waves is:

Pass a light beam through a polaroid which allows for only one mode of vibration. Now, rotate the Polaroid and observe the light coming out of the Polaroid.

(i) If the intensity of light is maximum only once after one complete rotation, then it is polarised light.

(ii) If the intensity is double the maximum, then the light is semi-polarised.

(iii) If the intensity is the same throughout the rotation, then the light is depolarised.

Polarisers are made primarily using minerals, in which the crystal orientation allows only a specific plane of vibration to pass through. The general rule is that electromagnetic vibrations are absorbed in a direction parallel to the alignment of the molecule.

Unpolarised light

Unpolarised Light is the light in which the vibrations of an electric vector are perpendicular to the direction of propagation of light waves and occur equally in all directions. The vibrations of light waves are symmetrical in all directions.

Polarised light

Polarised light is the light whose waves are perpendicular to the direction of propagation of light waves, but in only one direction. These are the light whose waves vibrate or oscillate in only one direction.

Methods of obtaining plane-polarised light: 

The following are the methods of obtaining plane-polarised light.

Polarisation of light by refraction:

When unpolarised light is incident at Brewster angle on a parallel plate of glass, the light reflected from both the top and bottom surfaces of the plate is entirely polarised. If we take a lot of similar plates of such glass, then the first parallel plate is thrown unpolarised light from the angle of polarisation, then the reflected part of the unpolarised light continues to polarise completely after reflection from the plates.

When the refracted part gradually passes beyond the plates, the amount of polarisation in it increases. In the end, if the number of plates is more, then the light emitted at the end becomes plane-polarised. This type of arrangement of Plato is called a bandage.

Polarisation by dichroism:

When unpolarised light passes through a crystal of a terminus, it splits into two plane-polarised rays. Tourmaline crystals selectively absorb either of these two refracted rays. The second ray goes out and plane-polarised light is obtained.

Polarisation by double refraction: When a ray of unpolarised light is incident on a crystal of calcite or Iceland spar, two refracted rays are obtained after refraction. This phenomenon of light is called double refraction. The crystals of calcite Iceland are called refractive crystals. The rays from a double refracting crystal are plane-polarised.

Examples of polarisation:

The light waves emitted from the light sources are used in daily life. Some examples of polarisation are electric bulbs, tube lights, etc., which are non-polar waves. Polaroids are of great use in our lives. For instance, they are used to avoid the glare of light reflected from water-soaked roads, excessive white light, bright floors, etc.

Three-dimensional pictures are seen in cinemas wearing Polaroid glasses. Similarly, polaroids are used in photography to determine the concentration of sugar in a solution and to study the optical properties of metals.

Polarisation of wave propagation

The electric field’s component propagates at the speed of light in a vacuum; therefore, the wave’s phase fluctuates in space and time while the polarisation state remains constant. When an electromagnetic wave interacts with matter, the material’s (complex) index of refraction affects the wave’s propagation. When a refractive index’s real or imaginary part depends on a wave’s polarisation state, qualities are known as birefringence and polarisation dichroism (or attenuation). The wave’s polarisation state is often altered.

Conclusion

In several parts of optical microscopy, light polarisation is particularly beneficial. A polarised light microscope is designed for viewing and photographing specimens that are visible primarily due to their anisotropic optical character. Anisotropic substances have physical properties that vary in how the light diffuses through them. To accomplish this task, a microscope must be fitted with both a polariser, placed in a light path somewhere in front of the template, and an analyser (a second polariser), placed in a visual path between the target background and view tubes or camera hole.

The ability of certain compounds to spin a plane of polarised light determines their chemical activity.