Can you please explain the physics behind this. Add comment. Sound and light are totally different physics I doubt if there is significant commonality. Light: Sound: propagates by pressure waves--no electric or magnetic fields involved. Will not propogate in a vacuum Light: Propogates by some combination of electric or magnetic fields Arago investigated the polarity of light originating from various sources in the sky and proposed a theory that predicted the velocity of light should decrease as it passes into a denser medium.
He also worked with Augustin Fresnel to investigate interference in polarized light and discovered that two beams of light polarized with their vibration directions oriented perpendicular to each other will not undergo interference.
Arago's polarizing filters, designed and built in , were made from a stack of glass sheets pressed together. A majority of the polarizing materials used today are derived from synthetic films invented by Dr. Edwin H. Land in , which soon overtook all other materials as the medium of choice for production of plane-polarized light. To produce the films, tiny crystallites of iodoquinine sulfate, oriented in the same direction, are embedded in a transparent polymeric film to prevent migration and reorientation of the crystals.
Land developed sheets containing polarizing films that are marketed under the trade name of Polaroid a registered trademark , which has become the accepted generic term for these sheets. Any device capable of selecting plane-polarized light from natural non-polarized white light is now referred to as a polar or polarizer , a name first introduced in by A. Because these filters are capable of differentially transmitting light rays, depending upon their orientation with respect to the polarizer axis, they exhibit a form of dichroism , and are often termed dichroic filters.
Polarized light microscopy was first introduced during the nineteenth century, but instead of employing transmission-polarizing materials, light was polarized by reflection from a stack of glass plates set at a degree angle to the plane of incidence. Later, more advanced instruments relied on a crystal of doubly refracting material such as calcite specially cut and cemented together to form a prism.
A beam of white non-polarized light entering a crystal of this type is separated into two components that are polarized in mutually perpendicular orthogonal directions. One of the light rays emerging from a birefringent crystal is termed the ordinary ray , while the other is called the extraordinary ray. The ordinary ray is refracted to a greater degree by electrostatic forces in the crystal and impacts the cemented surface at the critical angle of total internal reflection.
As a result, this ray is reflected out of the prism and eliminated by absorption in the optical mount. The extraordinary ray traverses the prism and emerges as a beam of linearly-polarized light that is passed directly through the condenser and to the specimen positioned on the microscope stage. Several versions of prism-based polarizing devices were once widely available, and these were usually named after their designers.
The most common polarizing prism illustrated in Figure 5 was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interactions between polarized light and crystalline substances.
Presented in Figure 5 is an illustration of the construction of a typical Nicol prism. A crystal of doubly refracting birefringent material, usually calcite, is cut along the plane labeled a-b-c-d and the two halves are then cemented together to reproduce the original crystal shape. A beam of non-polarized white light enters the crystal from the left and is split into two components that are polarized in mutually perpendicular directions.
One of these beams labeled the ordinary ray is refracted to a greater degree and impacts the cemented boundary at an angle that results in its total reflection out of the prism through the uppermost crystal face.
The other beam extraordinary ray is refracted to a lesser degree and passes through the prism to exit as a plane-polarized beam of light. Other prism configurations were suggested and constructed during the nineteenth and early twentieth centuries, but are currently no longer utilized for producing polarized light in modern applications.
Nicol prisms are very expensive and bulky, and have a very limited aperture, which restricts their use at high magnifications. Instead, polarized light is now most commonly produced by absorption of light having a set of specific vibration directions in a filter medium such as polarizing sheets where the transmission axis of the filter is perpendicular to the orientation of the linear polymers and crystals that comprise the polarizing material.
In modern polarizers, incident light waves having electric vector vibrations that are parallel to the crystal axis of the polarizer are absorbed. Many of the incident waves will have a vector orientation that is oblique, but not perpendicular to the crystal axis, and will only be partially absorbed. The degree of absorption for oblique light waves is dependent upon the vibration angle at which they impact the polarizer. Those rays that have angles close to parallel with respect to the crystal axis will be adsorbed to a much greater degree than those having angles close to the perpendicular.
The most common Polaroid filters termed the H-series transmit only about 25 percent of the incident light beam, but the degree of polarization of the transmitted rays exceeds 99 percent. A number of applications, most notably polarized optical microscopy, rely on crossed polarizers to examine birefringent or doubly refracting specimens.
When two polarizers are crossed, their transmission axes are oriented perpendicular to each other and light passing through the first polarizer is completely extinguished, or absorbed, by the second polarizer, which is typically termed an analyzer.
The light-absorbing quality of a dichroic polarizing filter determines exactly how much random light is extinguished when the polarizer is utilized in a crossed pair, and is referred to as the extinction factor of the polarizer. Quantitatively, the extinction factor is determined by the ratio of light that is passed by a pair of polarizers when their transmission axes are oriented parallel versus the amount passed when they are positioned perpendicular to each other.
In general, extinction factors between 10, and , are required to produce jet-black backgrounds and maximum observable specimen birefringence and contrast in polarized optical microscopy. The amount of light passing through a crossed pair of high-quality polarizers is determined by the orientation of the analyzer with respect to the polarizer.
When the polarizers are oriented perpendicular to each other, they display a maximum level of extinction. However, at other angles, varying degrees of extinction are obtained, as illustrated by the vector diagrams presented in Figure 6. The analyzer is utilized to control the amount of light passing through the crossed pair, and can be rotated in the light path to enable various amplitudes of polarized light to pass through.
In Figure 6 a , the polarizer and analyzer have parallel transmission axes and the electric vectors of light passing through the polarizer and analyzer are of equal magnitude and parallel to each other.
Rotating the analyzer transmission axis by degrees with respect to that of the polarizer reduces the amplitude of a light wave passing through the pair, as illustrated in Figure 6 b.
In this case, the polarized light transmitted through the polarizer can be resolved into horizontal and vertical components by vector mathematics to determine the amplitude of polarized light that is able to pass through the analyzer. The amplitude of the ray transmitted through the analyzer is equal to the vertical vector component illustrated as the yellow arrow in Figure 6 b. Continued rotation of the analyzer transmission axis, to a degree angle with respect to the transmission axis of the polarizer, further reduces the magnitude of the vector component that is transmitted through the analyzer Figure 6 c.
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:. 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. 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.
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. For example, the Stokes parameters are not independent in that. Stokes's paper appeared a dozen years before the publication of Maxwell's famous electromagnetic theory of light and 32 years before the publication of Poynting's work. Much was known about the properties of light waves even before they were fully grounded in an adequate theory.
Now we have to show that the Stokes parameters determine the ellipsometric parameters. In what follows fields are real. The real parts of the fields in Eq. The transformation from the original coordinate system to the primed coordinate system is Fig. Square and add the left sides of these equations and set the result equal to the sum of the squares of the right side:.
Now multiply Eq. Now square Eqs. If we go through the same steps for a left-circularly polarized wave, I, Q, and U are unchanged whereas V becomes. To recapitulate:. From Eq. The surfaces of constant phase and amplitude for the plane wave Eq. Thus the electric field of this wave occupies all space, which, of course, is physically unrealistic. To apply the previous analysis to real beams finite in lateral extent their properties have to be more or less laterally uniform.
The Stokes parameters [Eq. Nevertheless, once we know the form of these parameters we can devise feasible ways of measuring them with readily available linear retarders and polarizing filters see Prob. An electric wave described by Eq. Radiation from a microwave or radio antenna might closely fit this description because an antenna is a coherent object, its parts fixed relative to each other on the scale of the wavelength , driven by electric currents that are more or less time-harmonic.
It would take some ingenuity to make a microwave or radio antenna that did not radiate completely polarized waves. Radiation at much shorter wavelengths, however, often originates from vast arrays of tiny antennas molecules emitting more or less independently of each other, and hence we would not expect the same degree of regularity of the radiation from such sources. The extreme example of irregularity is unpolarized light whereas the extreme example of regularity is completely polarized light, both idealizations never strictly realized in nature.
But what is unpolarized light? Perhaps the simplest way to define such light is operationally, subject to previous caveats about ideal linear and circular polarizers. What kind of experimental tests can we devise to determine if a beam is unpolarized? Suppose that we transmit it through an ideal linear polarizer and discover that regardless of the orientation of its transmission axis, the transmitted irradiance is the same.
This implies that there is no preferred direction of the electric field, for if there were the irradiance would vary. But wait! A circularly-polarized beam would yield the same result. So we now have to determine if the beam exhibits a preferential handedness. First transmit the beam through an ideal left-circular polarizer, then through a right-circular polarizer. The Stokes parameters of partially polarized light also do not satisfy Eq. We can put more theoretical flesh onto these bare bones by extending Eq.
With this restriction the electric field Eq. The instantaneous Poynting vector corresponding to Eq. To determine the time average of Eq. We need consider only the first of these integrals because it sets the pattern for the other two. Divide the range of integration into N equal intervals At:.
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