The transverse nature of an electromagnetic wave is quite different from any other type of wave that has been discussed in The Physics Classroom Tutorial. Let's suppose that we use the customary slinky to model the behavior of an electromagnetic wave. As an electromagnetic wave traveled towards you, then you would observe the vibrations of the slinky occurring in more than one plane of vibration. This is quite different than what you might notice if you were to look along a slinky and observe a slinky wave traveling towards you. Indeed, the coils of the slinky would be vibrating back and forth as the slinky approached; yet these vibrations would occur in a single plane of space. That is, the coils of the slinky might vibrate up and down or left and right. Yet regardless of their direction of vibration, they would be moving along the same linear direction as you sighted along the slinky. If a slinky wave were an electromagnetic wave, then the vibrations of the slinky would occur in multiple planes. Unlike a usual slinky wave, the electric and magnetic vibrations of an electromagnetic wave occur in numerous planes. A light wave that is vibrating in more than one plane is referred to as unpolarized light. Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light. Such light waves are created by electric charges that vibrate in a variety of directions, thus creating an electromagnetic wave that vibrates in a variety of directions. This concept of unpolarized light is rather difficult to visualize. In general, it is helpful to picture unpolarized light as a wave that has an average of half its vibrations in a horizontal plane and half of its vibrations in a vertical plane.

Polarization of light by use of a Polaroid filter is often demonstrated in a Physics class through a variety of demonstrations. Filters are used to look through and view objects. The filter does not distort the shape or dimensions of the object; it merely serves to produce a dimmer image of the object since one-half of the light is blocked as it passed through the filter. A pair of filters is often placed back to back in order to view objects looking through two filters. By slowly rotating the second filter, an orientation can be found in which all the light from an object is blocked and the object can no longer be seen when viewed through two filters. What happened? In this demonstration, the light was polarized upon passage through the first filter; perhaps only vertical vibrations were able to pass through. These vertical vibrations were then blocked by the second filter since its polarization filter is aligned in a horizontal direction. While you are unable to see the axes on the filter, you will know when the axes are aligned perpendicular to each other because with this orientation, all light is blocked. So by use of two filters, one can completely block all of the light that is incident upon the set; this will only occur if the polarization axes are rotated such that they are perpendicular to each other.

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In the same manner, two Polaroid filters oriented with their polarization axes perpendicular to each other will block all the light. Now that's a pretty cool observation that could never be explained by a particle view of light.

It is possible to transform unpolarized light into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane. The process of transforming unpolarized light into polarized light is known as polarization. There are a variety of methods of polarizing light. The four methods discussed on this page are:

If the focal length of two lenses is the same, the lens with the larger diameter will be brighter. For example, if the focal length is 50mm and the lens ...

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An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different focal points.

Unfortunately, gratings suffer from the aberrations of concave mirrors and others due to their diffraction capabilities. Working in the Rowland conditions definitively limits the imaging quality of the instruments. The major aberration here is astigmatism. This aberration can be tolerated with a monochromator since only horizontal focusing is required to separate the wavelengths of spectrum.

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3. Consider the three pairs of sunglasses below. Identify the pair of glasses is capable of eliminating the glare resulting from sunlight reflecting off the calm waters of a lake? _________ Explain. (The polarization axes are shown by the straight lines.)

Distortion is the most easily recognized aberration as it deforms the image as whole. It arises from the unequal magnification of the peripheral part of a lens (or a mirror) from that of its central part. In “barrel distortion,” image magnification decreases with distance from the optical axis. In “pincushion distortion,” image magnification increases with the distance from the optical axis.

At normal incidence (zero order, λ3), the aberration is minimum and image is straight. But closer the images are from the grating, more stretched and curved are their images. This stretching can be severe, depends on the position of the image on the Rowland circle and hence of the observed wavelength. It results in both loss of signal and loss of resolution, especially in spectrograph mode when CCD detectors are used.

Spherically shaped lenses and mirrors share this problem. Parallel light rays that pass through the central region focus farther away than those that pass through the edges. The result is many focal points, which produces a blurry image.

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Polarization is also used in the entertainment industry to produce and show 3-D movies. Three-dimensional movies are actually two movies being shown at the same time through two projectors. The two movies are filmed from two slightly different camera locations. Each individual movie is then projected from different sides of the audience onto a metal screen. The movies are projected through a polarizing filter. The polarizing filter used for the projector on the left may have its polarization axis aligned horizontally while the polarizing filter used for the projector on the right would have its polarization axis aligned vertically. Consequently, there are two slightly different movies being projected onto a screen. Each movie is cast by light that is polarized with an orientation perpendicular to the other movie. The audience then wears glasses that have two Polaroid filters. Each filter has a different polarization axis - one is horizontal and the other is vertical. The result of this arrangement of projectors and filters is that the left eye sees the movie that is projected from the right projector while the right eye sees the movie that is projected from the left projector. This gives the viewer a perception of depth.

Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. The extent to which polarization occurs is dependent upon the angle at which the light approaches the surface and upon the material that the surface is made of. Metallic surfaces reflect light with a variety of vibrational directions; such reflected light is unpolarized. However, nonmetallic surfaces such as asphalt roadways, snowfields and water reflect light such that there is a large concentration of vibrations in a plane parallel to the reflecting surface. A person viewing objects by means of light reflected off of nonmetallic surfaces will often perceive a glare if the extent of polarization is large. Fishermen are familiar with this glare since it prevents them from seeing fish that lie below the water. Light reflected off a lake is partially polarized in a direction parallel to the water's surface. Fishermen know that the use of glare-reducing sunglasses with the proper polarization axis allows for the blocking of this partially polarized light. By blocking the plane-polarized light, the glare is reduced and the fisherman can more easily see fish located under the water.

Long focal length lenses frequently feature tiny depths of field, allowing them to focus close-up on little objects—even those in the distance. Conversely ...

A light wave is an electromagnetic wave that travels through the vacuum of outer space. Light waves are produced by vibrating electric charges. The nature of such electromagnetic waves is beyond the scope of The Physics Classroom Tutorial. For our purposes, it is sufficient to merely say that an electromagnetic wave is a transverse wave that has both an electric and a magnetic component.

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Referring to the above question, the glare is the result of a large concentration of light aligned parallel to the water surface. To block such plane-polarized light, a filter with a vertically aligned polarization axis must be used.

An optical aberration is a departure of the performance of an optical system from the predictions of paraxial optics. In the existence of an optical aberration, light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system. Optical aberrations fall into two classes: monochromatic and chromatic.

2. Light becomes partially polarized as it reflects off nonmetallic surfaces such as glass, water, or a road surface. The polarized light consists of waves vibrate in a plane that is ____________ (parallel, perpendicular) to the reflecting surface.

The optimization of the image correction of gratings can be calculated for a better image quality on the optical axis of the instrument (monochromator layout) or on a focal plane (spectrograph layout). In the last case, the optimization enlarges the focal plane, the grating of the spectrograph works in fixed position and the wavelength range selection is achieved by sliding the detector in the focal plane of the instrument. The correction is excellent in both cases.

GCxGC (GC by GC), also known as Comprehensive Two-Dimensional Gas Chromatography, is a powerful analytic technique that utilizes two columns of differing ...

The first filter will polarize the light, blocking one-half of its vibrations. The second filter will have no affect on the light. Being aligned parallel to the first filter, the second filter will let the same light waves through.

Our model of the polarization of light provides some substantial support for the wavelike nature of light. It would be extremely difficult to explain polarization phenomenon using a particle view of light. Polarization would only occur with a transverse wave. For this reason, polarization is one more reason why scientists believe that light exhibits wavelike behavior.

A master grating is an original unit recorded as a unique piece. A master grating can be utilized as the “mother” of multiples copies, called replicas.

A VLS grating is one whose grooves, when projected onto the tangent plane, form a set of straight parallel lines whose spacing varies from groove to groove. Varying the groove spacing across the surface of the grating moves the tangential focal curve, while keeping the grooves straight and parallel keeps the sagittal focal curve fixed. It corrects for spherical aberration associated with conventional spherical gratings. The VLS technique can also be applied on toroidal grating for an optimum correction.

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Most of the time, VUV monochromators are preferred when equipped with master gratings. But unfortunately, such gratings are extremely expensive and have a long delivery time.

The spectroscopic images can be improved by using toroidal gratings. A toroidal grating is a form of an elliptic paraboloid with different vertical and horizontal focal distances. It reduces the stretching and the curvature of astigmatism.

A picket-fence analogy is often used to explain how this dual-filter demonstration works. A picket fence can act as a polarizer by transforming an unpolarized wave in a rope into a wave that vibrates in a single plane. The spaces between the pickets of the fence will allow vibrations that are parallel to the spacings to pass through while blocking any vibrations that are perpendicular to the spacings. Obviously, a vertical vibration would not have the room to make it through a horizontal spacing. If two picket fences are oriented such that the pickets are both aligned vertically, then vertical vibrations will pass through both fences. On the other hand, if the pickets of the second fence are aligned horizontally, then the vertical vibrations that pass through the first fence will be blocked by the second fence. This is depicted in the diagram below.

1. Suppose that light passes through two Polaroid filters whose polarization axes are parallel to each other. What would be the result?

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Some aberration affects the images due to off axis rays. The image of a spot looks like several cylinders not centered, like an image of a “comet”, where its name comes from.

Two basic types of spectrographs and monochromators are used in the VUV: normal incidence instruments a better design for 100-400 nm, and grazing incidence instruments for 2-100 nm.

Monochromatic aberrations are caused by the geometry of the lens and occur both when light is reflected and when it is refracted. Monochromatic aberrations include spherical aberration, coma, astigmatism, and field curvature and image distortion.

The realization of a real VUV monograph has to be done without a toroidal grating. The Plane Grating Spectrograph (PGS) configuration is one of the best choices. The PGS layout operates with a toroid mirror and a plane grating working at a grazing angle. It also has the advantage of being more affordable grating as they have a plane design.

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Polarization also occurs when light is scattered while traveling through a medium. When light strikes the atoms of a material, it will often set the electrons of those atoms into vibration. The vibrating electrons then produce their own electromagnetic wave that is radiated outward in all directions. This newly generated wave strikes neighboring atoms, forcing their electrons into vibrations at the same original frequency. These vibrating electrons produce another electromagnetic wave that is once more radiated outward in all directions. This absorption and reemission of light waves causes the light to be scattered about the medium. (This process of scattering contributes to the blueness of our skies, a topic to be discussed later.) This scattered light is partially polarized. Polarization by scattering is observed as light passes through our atmosphere. The scattered light often produces a glare in the skies. Photographers know that this partial polarization of scattered light leads to photographs characterized by a washed-out sky. The problem can easily be corrected by the use of a Polaroid filter. As the filter is rotated, the partially polarized light is blocked and the glare is reduced. The photographic secret of capturing a vivid blue sky as the backdrop of a beautiful foreground lies in the physics of polarization and Polaroid filters.

The design of VUV instruments suffers from the constraints of VUV optical materials. Transmission through bulk materials is limited to λ < 105 nm, the short wavelength transmission is limited of LiF or λ < 115 nm for MgF2. Reflective configuration is used in a VUV optical layout. However, reflectance from metal surfaces also decreases at short wavelengths. Several coating materials are introduced to increase reflectivity, such as Al, Os, Pt, Au, Rh and Ir. Above 120 nm, the main broadband reflector for VUV wavelengths is Al with MgF2 coating, having a normal incidence reflectivity of up to 90% under certain conditions. Os, Pt, Au, and Ir have a reflectance of about 60% from 5-200 nm in grazing configuration.

Figure 2.1 : Principle scheme of an optical microscope. The objective lens has a much shorter focal length than the eye-piece, in order to magnify the ...

Polarization can also occur by the refraction of light. Refraction occurs when a beam of light passes from one material into another material. At the surface of the two materials, the path of the beam changes its direction. The refracted beam acquires some degree of polarization. Most often, the polarization occurs in a plane perpendicular to the surface. The polarization of refracted light is often demonstrated in a Physics class using a unique crystal that serves as a double-refracting crystal. Iceland Spar, a rather rare form of the mineral calcite, refracts incident light into two different paths. The light is split into two beams upon entering the crystal. Subsequently, if an object is viewed by looking through an Iceland Spar crystal, two images will be seen. The two images are the result of the double refraction of light. Both refracted light beams are polarized - one in a direction parallel to the surface and the other in a direction perpendicular to the surface. Since these two refracted rays are polarized with a perpendicular orientation, a polarizing filter can be used to completely block one of the images. If the polarization axis of the filter is aligned perpendicular to the plane of polarized light, the light is completely blocked by the filter; meanwhile the second image is as bright as can be. And if the filter is then turned 90-degrees in either direction, the second image reappears and the first image disappears. Now that's pretty neat observation that could never be observed if light did not exhibit any wavelike behavior.

Polarization has a wealth of other applications besides their use in glare-reducing sunglasses. In industry, Polaroid filters are used to perform stress analysis tests on transparent plastics. As light passes through a plastic, each color of visible light is polarized with its own orientation. If such a plastic is placed between two polarizing plates, a colorful pattern is revealed. As the top plate is turned, the color pattern changes as new colors become blocked and the formerly blocked colors are transmitted. A common Physics demonstration involves placing a plastic protractor between two Polaroid plates and placing them on top of an overhead projector. It is known that structural stress in plastic is signified at locations where there is a large concentration of colored bands. This location of stress is usually the location where structural failure will most likely occur. Perhaps you wish that a more careful stress analysis were performed on the plastic case of the CD that you recently purchased.

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Aberration leads to blurring of the image produced by an image-forming optical system. Makers of optical instruments need to correct optical systems to compensate for aberration.

The most common method of polarization involves the use of a Polaroid filter. Polaroid filters are made of a special material that is capable of blocking one of the two planes of vibration of an electromagnetic wave. (Remember, the notion of two planes or directions of vibration is merely a simplification that helps us to visualize the wavelike nature of the electromagnetic wave.) In this sense, a Polaroid serves as a device that filters out one-half of the vibrations upon transmission of the light through the filter. When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane; it emerges as polarized light.

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A Polaroid filter is able to polarize light because of the chemical composition of the filter material. The filter can be thought of as having long-chain molecules that are aligned within the filter in the same direction. During the fabrication of the filter, the long-chain molecules are stretched across the filter so that each molecule is (as much as possible) aligned in say the vertical direction. As unpolarized light strikes the filter, the portion of the waves vibrating in the vertical direction are absorbed by the filter. The general rule is that the electromagnetic vibrations that are in a direction parallel to the alignment of the molecules are absorbed.

S: Spot source (or slit of the instrument) λ3: Zero order position λ1, λ2: Dispersed wavelength positions λ1> λ2 i: incidence angle r: reflectance angle

Chromatic aberration is a type of distortion in which the lens fails to focus all colors on the same convergence point, due to the dispersion of the lens (different refractive index of the lens for different wavelengths of light).

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The alignment of these molecules gives the filter a polarization axis. This polarization axis extends across the length of the filter and only allows vibrations of the electromagnetic wave that are parallel to the axis to pass through. Any vibrations that are perpendicular to the polarization axis are blocked by the filter. Thus, a Polaroid filter with its long-chain molecules aligned horizontally will have a polarization axis aligned vertically. Such a filter will block all horizontal vibrations and allow the vertical vibrations to be transmitted (see diagram above). On the other hand, a Polaroid filter with its long-chain molecules aligned vertically will have a polarization axis aligned horizontally; this filter will block all vertical vibrations and allow the horizontal vibrations to be transmitted.

A Rowland demonstrated that the dispersed spectrum of an illuminated point lying on a circle is focused on this circle, if the following setup is respected (see figure). A lot of VUV monochromators use this design.

In order to limit the number of reflections onto optics, concave gratings are often used as single element in VUV spectrometers.