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  Polarization of Light, Physics tutorial

Polarized and Unpolarized Light:

Light of the single color can be explained as the wave with specified wavelength or as photon with the specified energy. Another feature of light is that it can be polarized with wave vibrations lying in one plane. Several materials respond differently in their skill to transmit light depending on relation of the plane of polarization to crystal axes of the materials. In optical microscopy, light beams can be polarized using filters.

Few natural crystals like, tourmaline permits rays of light vibrating in certain direction to pass through and block other rays vibrating in other direction. If the observer view light coming from the source with two of such crystals A and B as arranged in Figure the observer would note positions of brightness and darkness as crystal B is rotated. You would notice that at a stage a very bright light would get in his eyes but at another stage you would not see anything. Thus, you would notice positions of maximum brightness and maximum darkness as crystal rotates near the eyes. Position of maximum brightness takes place when planes or crystals through which light is permitted to pass are oriented in same direction. On the other hand, the position of maximum (complete) darkness takes place when the planes are at right angles to each other. Artificial crystalline materials which polarize light are known as polaroids.

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 Optical Activity:

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Certain substances have the skill to rotate plane of polarized light when the plane polarized light is passed through them. Such substances are said to be Optically Active. If rotation of plane is clockwise i.e., to right, then substance is dextrorotatory. On the other hand, it is levorotatory if the rotation is anticlockwise. Degree of rotation may be determined by means of the polarimeter. In simple form, polarimeter comprises of two Polaroid sheets functioning as polarizer and analyzer and tube containing the substance. With tube empty, maximum amount of light reaches to eye when sheets are oriented in same direction. Analyzer is then turned through 90o before placing substance in position. Depending on rotation of plane of polarization analyzer would appear brighter.

Analyzer is again rotated until minimum light is observed. Difference in readings of analyzer provides the angle of rotation of beam.

Methods of Polarization:

i) Polarization by Reflection:

Unpolarized light can also undergo polarization by reflection of nonmetallic surfaces. Extent to which polarization takes place is dependent on angle at which light approaches surface and upon material that the surface is made of. Metallic surfaces reflect light with the diversity of vibrational directions; such reflected light is unpolarized. Though, nonmetallic surfaces like asphalt roadways, snowfields and water reflect light such that there is a large concentration of vibrations in the plane parallel to reflecting surface.

ii) Polarization by Refraction:

Polarization can also take place by refraction of light. Refraction takes place when the beam of light passes from one material to another material.

At surface of two materials, path of beam changes its direction. Refracted beam gets some degree of polarization. Most frequently, polarization takes place in the plane perpendicular to surface. Polarization of refracted light is frequently shown using the unique crystal which acts as a double-refracting crystal. Iceland spar, a quite uncommon form of mineral calcite, refracts incident light in two different paths. Light is split in two beams upon entering crystal. Then, if the object is observed by looking through Iceland spar crystal, two images will be observed. Two images are the result of double refraction of light. Both refracted light beams are polarized - one in the direction parallel to surface and other in the direction perpendicular to surface. As these two refracted rays are polarized with the perpendicular orientation, the polarizing filter can be utilized to entirely block one of the images.

iii) Double refraction:

Double refraction, also known as birefringence,  an optical property in which the single ray of unpolarized light entering the anisotropic medium is split in two rays, each traveling in the different direction. One ray (known as extraordinary ray) is bent, or refracted, at angle as it travels through the medium; other ray (called the ordinary ray) passes through medium unchanged.

Double refraction can be seen by comparing two materials, glass and calcite. If the pencil mark is drawn upon the sheet of paper and then covered with the piece of glass, only one image will be observed; but if same paper is covered with the piece of calcite, and crystal is oriented in the specific direction, then two marks will become visible.

In double refraction, ordinary ray and extraordinary ray are polarized in planes vibrating at right angles to each other. Furthermore, refractive index (a number that fins out the angle of bending specific for every medium) of ordinary ray is seen to be constant in all directions; refractive index of extraordinary ray differs according to direction taken as it has components which are both parallel and perpendicular to crystal's optic axis. As speed of light waves in the medium is equal to their speed in the vacuum divided by index of refraction for that wavelength, the extraordinary ray can move either faster or slower than ordinary ray.

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