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Linearly polarizedlight

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

There are hundreds of unique objective lenses to choose from, but once you have a greater understanding of the most common types, you can make a more informed decision regarding which lens is right for you. Whether you are a hobbyist or whether you require the use of a microscope in your day-to-day research, it’s important to gain an understanding of the strengths and weaknesses across the spectrum of objective lenses. Once you know exactly what you’re looking for, you’ll be well on your way to obtaining the best results and having an optimal viewing experience.

P polarised lightmeaning

One of the most common uses of polarization today is the liquid crystal display (LCD) used in numerous applications including wrist watches, computer screens, timers, clocks, and many others. These devices are based upon the interaction of rod-like liquid crystalline molecules with an electric field and polarized light waves. The liquid crystalline phase exists in a ground state that is termed cholesteric where the molecules are oriented in layers where each successive layer is slightly twisted to form a spiral pattern. When polarized light waves interact with the liquid crystalline phase the wave is "twisted" by an angle of approximately 90 degrees with respect to the incident wave. This angle is a function of the helical pitch of the cholesteric liquid crystalline phase, which is dependent upon the chemical composition of the molecules (it can be fine-tuned by small changes to the molecules).

s-polarization vsppolarization

An excellent example of the basic application of liquid crystals to display devices can be found in the seven-segment LCD numerical display (Figure 3). Here, the liquid crystalline phase is sandwiched between two glass plates that have electrodes attached similar to those depicted in the illustration below. In figure 3, the glass plates are drawn with seven black electrodes that can be individually charged (these electrodes are transparent to light in real devices). Light passing through polarizer 1 is polarized in the vertical direction and, when no current is applied to the electrodes, the liquid crystalline phase induces a 90 degree "twist" of the light and it can pass through polarizer 2, which is polarized horizontally and is perpendicular to polarizer 1. This light can then form one of the seven segments on the display.

Plane polarizedlight

This concept is illustrated in Figure 1 below, and we have also constructed an interactive Java tutorial that explores the interaction of light waves with polarizers. In this example, the incident light electric field vectors are vibrating perpendicular to the direction of propagation in an equal distribution of all planes before encountering the first polarizer. The polarizers illustrated above are actually filters containing long-chain polymer molecules that are oriented in a single direction. Only the incident light that is vibrating in the same plane as the oriented polymer molecules is absorbed, while light vibrating at right angles to the plane is passed through the first polarizing filter. In Figure 1, polarizer 1 is oriented vertically to the incident beam so it will pass only the waves that are vertical in the incident beam. The wave passing through polarizer 1 is subsequently blocked by polarizer 2 because the second polarizer is oriented horizontally with respect to the electric field vector in the light wave. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the practice of polarized light microscopy.

This type of polarized light is often termed glare and can be easily demonstrated by viewing the distant part of a highway on a sunny day. The reflection and polarization of light according to the Brewster theory can be more thoroughly examined with our Brewster Angle Java Tutorial. Light reflected by the flat surface of a highway is partially polarized with the electric field vectors vibrating in a direction that is parallel to the ground. This light can be blocked by polarizing filters oriented in a vertical direction as illustrated below in Figure 2 with a pair of polarized sunglasses.

Circular polarization

Infinity objective lenses did not become common until the 1980s but have since carved out a permanent spot in the microscope objective market. Previously, all microscopes had a standard tube length–the distance from the eyepiece to the objective lens was always 160 mm. Once microscope manufacturers began developing microscopes with varying tube lengths, lens manufacturers had to catch up with the changing technology. New tube lengths meant that microscopy equipment developers needed to adjust for these changes in their accessories, including objective lenses. Infinity optical systems use multiple sets of lenses within the lens house to correct a wide range of tube lengths–typically from 160-200 mm. This enables the lenses to be more versatile between microscopes of varying tube lengths.

Achromatic lenses are used to diminish chromatic and spherical aberrations which are the loss of color and focus that can happen when light wavelengths refract in direct light. These aberrations can be controlled by using an objective lens that contains both a convex and concave lens inside. Mounting these two different types of lenses to each other can bring wavelengths of red and blue light closer together, which puts them in the same focus and cancels out chromatic aberration. Another type of lens used to correct for both color and spherical aberration is the plan (or planar) lens. These produce a flatter field and can also give you a much larger working distance. However, they can be more expensive than achromatic lenses, so choosing between the two depends largely on how much power you need in your objective lens, and whether or not you need to adjust for field curvature, which only plan lenses can do. Achromatic lenses and plan lenses both come in dozens of magnifications and types, accommodating a wide variety of microscopy needs.

When current is applied to the electrodes, the liquid crystalline phase aligns with the current and loses the cholesteric spiral pattern. Light passing through a charged electrode is not twisted and is blocked by polarizer 2. By coordinating the voltage on the seven positive and negative electrodes, the display is capable of rendering the numbers 0 through 9. In this example the upper right and lower left electrodes are charged and block light passing through them, allowing formation of the number "2".

Circularly polarizedlight

Obtaining high-contrast images of transparent specimens is difficult, especially when your specimen is alive and moving on a slide. Phase-contrast lenses allow you to observe microorganisms without having to fix and stain them. When your specimens are kept alive, a variety of biological functions can be examined and analyzed in real-time. Phase plates at the top of the objective lens diffract light, allowing these specialized lenses to tap into tiny changes in wavelength amplitude, which appears to the viewer as starker contrast on the slide. This makes the specimen much easier to view and observe.

Unpolarizedlight

Unpolarized incident light (natural sunlight, for example) is polarized to a certain degree when it is reflected from an insulating surface like water or a highway. In this case, light waves that have the electric field vectors parallel to the surface are reflected to a greater degree than those with different orientations. The optical properties of the insulating surface determine the exact amount of reflected light that is polarized. Mirrors are not good polarizers, although many transparent materials will be very good polarizers, but only if the incident light angle is within certain limits. In this case, the particular angle inducing maximum polarization is known as the Brewster angle given by the expression:

Natural sunlight and almost every other form of artificial illumination transmits light waves whose electric field vectors vibrate in all perpendicular planes with respect to the direction of propagation. When the electric field vectors are restricted to a single plane by filtration, then the light is said to be polarized with respect to the direction of propagation and all waves vibrate in the same plane.

Specialized microscopes, such as metallurgical microscopes, require their own specific metallurgical objective lenses. These devices are most often used to examine structural detail of ceramics, metals and other non-living materials. Another common microscope objective accessory is a Barlow lens. These can be added to the bottom of an objective lens to either increase or decrease its working distance, field of view or magnification. Since they can be interchanged between lenses, they are a cost-effective way to change the power and magnification of lenses you already own. Lastly, if all these lenses are starting to seem overwhelming, remember one quick trick for determining magnification at a glance: look at the band of color near the bottom of your objective lens. While the magnification number is usually written right on the lens, you can also quickly determine its strength by the color ring. Red indicates 5x magnification, while yellow means 10x, light blue means 40x and white can mean 100-250x.

The objective lens is the most important optical component of the microscope. It’s the part that sits in closest proximity to the specimen being examined, gathering light to produce optimal images for observation and analysis. This lens creates the first magnification by spreading out the light’s rays to make the object appear considerably larger by the time it meets your field of view at the other end of the eyepiece. Such a critical piece of equipment doesn’t come in a one-size-fits-all package. Below, we will discuss some of the different types of microscope objective lenses and the unique roles they play in microscopy.

Polarization of light is very useful in many aspects of optical microscopy. The microscope configuration uses crossed polarizers where the first polarizer (termed: the polarizer) is placed below the sample in the light path and the second polarizer (termed: the analyzer) is placed above the sample, between the objective and the eyepieces. With no sample on the microscope stage, the light polarized by the polarizer is blocked by the analyzer and no light is visible. When samples that are birefringent are viewed on the stage between crossed polarizers, the microscopist can visualize aspects of the samples through light rotated by the sample and then able to pass through the analyzer. The details of polarized light microscopy are thoroughly discussed in our microscopy section of this primer.

The lenses of the sunglasses have polarizing filters that are oriented vertically with respect to the frames. In the Figure 2 above, the blue light waves have their electric field vectors oriented in the same direction as the polarizing lenses and, thus, are passed through. In contrast, the red light wave is perpendicular to the filters and is blocked by the lenses. Polarizing sunglasses are very useful when driving in the sun or at the beach where sunlight is reflected from the surface of the road or water leading to glare that can be almost blinding.