Polarized Light
The vibration motion of a light wave is perpendicular, or nearly perpendicular, to the direction it is propagating. In normal unpolarized beams of light, waves vibrate in many different directions, shown by arrows in Figure 1b. However, we can filter or alter a light beam to make all the waves vibrate in one direction parallel to a particular plane (shown by arrows in Figure 1c).
Properties of light rays:
The light is then plane polarized, sometimes called just polarized. Light becomes polarized in different ways. Reflection from a shiny surface can partially or completely polarize light because light vibrating in planes parallel to the reflecting surface is especially well reflected, while light vibrating in other directions is absorbed. This is why sunglasses with polarizing lenses help eliminate glare. Suppose light passes through a polarizing f ilter that constrains it to vibrate in a north-south (up-down) direction. The polarized beam, although perhaps decreased in intensity, appears the same to our eyes because human eyes cannot determine whether light is polarized. If, however, another polarizing filter is in the path of the beam, we can easily determine that the beam is polarized (Figure 2). If the second filter allows only light vibrating in a north-south direction to pass, the polarized beam will pass through it (Figure 2a). If we slowly rotate the second filter to an east-west direction, it will gradually transmit less light, and eventually no light (Figure 2b).
polarizing filters :
Polarizing Microscopes
Polarizing microscopes, also called petrographic microscopes, are in many respects the same as other microscopes (Figures 3 and 4).
They magnify small objects so we can see them in greater detail. A bulb provides a white light source. The light passes through several filters and diaphragms before it reaches the stage and interacts with the material being observed. One of the most important f ilters is the lower polarizer, which ensures that all light striking samples on the stage is plane polarized (vibrating, or having wave motion, in only one plane). The presence of a lower polarizer sets polarizing microscopes apart from others. In most modern polarizing microscopes, the lower polarizer only allows light vibrating in an east-west direction to reach the stage. Older microscopes, however, have the lower polarizer oriented in a north-south direction. A fixed condensing lens and a diaphragm in the substage help concentrate light on the sample. For most purposes, we use orthoscopic illumination, in which an unfocused beam travels from the substage through the sample and straight up the microscope tube. The light rays travel orthogonal to the stage and to a sample or thin section on the stage. However, we can insert a special lens a conoscopic lens—between the lower polarizer and stage to produce conoscopic illumination when needed (Figure 4).
Orthoscopic and Conoscopic Illumination :
The conoscopic lens, also called a condenser lens, causes the light beam to converge (focus) on a small spot on the sample and illuminates the sample with a cone of nonparallel rays.
We can rotate the microscope stage to change the orientation of the sample relative to the polarized light. Because most minerals are anisotropic, the interaction of the light with a mineral varies with stage rotation. A calibrated angular scale allows us to make precise measurements of crystal orientation. The scale is also useful for measuring angles between cleavages, crystal faces, and twin orientations, and for measuring other optical properties.
Above the stage, a rotating turret holds several objective lenses. They usually range in magnification from about 2 x to 5 x . Different objective lenses can have different numerical apertures (N.A.), a value that describes the angles at which light can enter a lens, which is an important consideration when making certain measurements. In the discussion of interference figures. We have assumed that the objective lens being used has an N.A. of 0.85, since this is by far the most common today. If you use a lens with a different N.A., some of the given angular values may be in error. The ocular, an additional lens usually providing 8 x or 10 x magnification, is in the eyepiece. Binocular microscopes, such as the one in Figure 1, have two eyepieces and two oculars. Oculars have crosshairs that aid in making angular measurements when we rotate the stage. The total magnification, which is the product of the objective lens magnification and the ocular magnification, varies from about 16 x lenses used.
We can insert several other filters and lenses between the objective lens and the ocular when needed (Figure 4). The upper polarizer, sometimes called the analyzer, is a polarizing filter oriented at 90⁰ to the lower polarizer, which we can insert or remove from the path of the light beam. If no sample is on the stage, light that passes through the lower polarizer cannot pass through the upper polarizer. If a sample is on the stage, it usually changes the polarization of the light so that some can pass through the upper polarizer. We can also insert an accessory plate above the upper polarizer. The most common kind of accessory plate used today is called a “full wave” plate. In the past, all full wave plates were made of gypsum and are still often referred to as “gypsum plates,” but today they are made of quartz. Above the accessory plate, most polarizing microscopes have a Bertrand lens and diaphragm. We use them with the substage conoscopic lens to view minerals in conoscopic illumination, allowing us to make some special kinds of measurements.
Petrologists and mineralogists use polarizing microscopes with or without the upper polarizer inserted . Without the upper polarizer, we see the sample in plane polarized light (PPlight) ; with the upper polarizer, we see it in crossed polars (XPlight). Grain size, shape, color, cleavage, and other physical properties are best revealed in PP light. The optical properties refractive index and pleochroism are also determined using PP light. We use XP light, sometimes focused with conoscopic and Bertrand lenses, to determine properties including retardation, optic sign, and 2V. These properties are discussed in detail later.
We examine minerals or rocks in grain mounts or in thin sections(Figure 5).
For determining some mineral properties, a small amount of a powdered mineral sample is placed on a glass slide to produce a grain mount. The grains must be thin enough so that light can pass through them without a significant loss of intensity, usually 0.10 to 0.15 mm in longest dimension. A small amount of liquid (often referred to as a refractive index oil) surrounds them, and a thin piece of glass, called a cover slip, is placed over the grains and liquid. Grain mounts and refractive index oils are absolutely necessary for making some types of measurements. Petrologists use thin sections, however, for routine mineral identification and other petrographic work.