Polarization of Light. Introduction. Light, viewed classically, is a transverse electromagnetic wave. Namely, the underlying os- cillation (in this case oscillating. Lecture Polarisation of light, introduction. Lecture aims to explain: 1. Light as a transverse electro-magnetic wave. 2. Importance of polarisation of light. 3. Polarisation of Light. Safety hazards and precautions: • This practical uses a class 2 laser. Do not look directly into the beam, or at any reflections. You should not.
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polarised light, unpolarised light, randomly polarised light, linear polarisation ( Describe and discuss various applications of polarised light and explain how. Polarizers. Reading – Shen and Kong – Ch. 3. Outline. Review of the Lorentz Oscillator. Reflection of Plasmas and Metals. Polarization of Scattered Light. Originally an empirical law, nowadays Malus' law is seen as a key experiment to demonstrate the transverse nature of electromagnetic waves, as well as the.
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.
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. 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.
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.
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.
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.
Polarization by Reflection 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.
It was this effect that provided the first discovery of polarization, by Erasmus Bartholinus in Media in which transmission of one polarization mode is preferentially reduced are called dichroic or diattenuating. Like birefringence, diattenuation can be with respect to linear polarization modes in a crystal or circular polarization modes usually in a liquid.
Polarization by Reflection
Devices that block nearly all of the radiation in one mode are known as polarizing filters or simply " polarizers ". The output of an ideal polarizer is a specific polarization state usually linear polarization with an amplitude equal to the input wave's original amplitude in that polarization mode. Power in the other polarization mode is eliminated. However, in many instances the more relevant figure of merit is the polarizer's degree of polarization or extinction ratio , which involve a comparison of g 1 to g 2.
In addition to birefringence and dichroism in extended media, polarization effects describable using Jones matrices can also occur at reflective interface between two materials of different refractive index. These effects are treated by the Fresnel equations. Part of the wave is transmitted and part is reflected; for a given material those proportions and also the phase of reflection are dependent on the angle of incidence and are different for the s and p polarizations.
Therefore, the polarization state of reflected light even if initially unpolarized is generally changed. Any light striking a surface at a special angle of incidence known as Brewster's angle , where the reflection coefficient for p polarization is zero, will be reflected with only the s -polarization remaining. This principle is employed in the so-called "pile of plates polarizer" see figure in which part of the s polarization is removed by reflection at each Brewster angle surface, leaving only the p polarization after transmission through many such surfaces.
The generally smaller reflection coefficient of the p polarization is also the basis of polarized sunglasses ; by blocking the s horizontal polarization, most of the glare due to reflection from a wet street, for instance, is removed.
In the important special case of reflection at normal incidence not involving anisotropic materials there is no particular s or p polarization. Both the x and y polarization components are reflected identically, and therefore the polarization of the reflected wave is identical to that of the incident wave. However, in the case of circular or elliptical polarization, the handedness of the polarization state is thereby reversed, since by convention this is specified relative to the direction of propagation.
But in the general case of reflection at a nonzero angle of incidence, no such generalization can be made. For instance, right-circularly polarized light reflected from a dielectric surface at a grazing angle, will still be right-handed but elliptically polarized.
Linear polarized light reflected from a metal at non-normal incidence will generally become elliptically polarized. These cases are handled using Jones vectors acted upon by the different Fresnel coefficients for the s and p polarization components. Some optical measurement techniques are based on polarization.
In many other optical techniques polarization is crucial or at least must be taken into account and controlled; such examples are too numerous to mention. In engineering , the phenomenon of stress induced birefringence allows for stresses in transparent materials to be readily observed.
As noted above and seen in the accompanying photograph, the chromaticity of birefringence typically creates colored patterns when viewed in between two polarizers.
As external forces are applied, internal stress induced in the material is thereby observed. Additionally, birefringence is frequently observed due to stresses "frozen in" at the time of manufacture. This is famously observed in cellophane tape whose birefringence is due to the stretching of the material during the manufacturing process.
Ellipsometry is a powerful technique for the measurement of the optical properties of a uniform surface. It involves measuring the polarization state of light following specular reflection from such a surface.
This is typically done as a function of incidence angle or wavelength or both. Since ellipsometry relies on reflection, it is not required for the sample to be transparent to light or for its back side to be accessible. Ellipsometry can be used to model the complex refractive index of a surface of a bulk material.
It is also very useful in determining parameters of one or more thin film layers deposited on a substrate. Due to their reflection properties , not only are the predicted magnitude of the p and s polarization components, but their relative phase shifts upon reflection, compared to measurements using an ellipsometer. A normal ellipsometer does not measure the actual reflection coefficient which requires careful photometric calibration of the illuminating beam but the ratio of the p and s reflections, as well as change of polarization ellipticity hence the name induced upon reflection by the surface being studied.
In addition to use in science and research, ellipsometers are used in situ to control production processes for instance. The property of linear birefringence is widespread in crystalline minerals , and indeed was pivotal in the initial discovery of polarization.
In mineralogy , this property is frequently exploited using polarization microscopes , for the purpose of identifying minerals. See optical mineralogy for more details. Sound waves in solid materials exhibit polarization. Differential propagation of the three polarizations through the earth is a crucial in the field of seismology. Horizontally and vertically polarized seismic waves shear waves are termed SH and SV, while waves with longitudinal polarization compressional waves are termed P-waves.
We have seen above that the birefringence of a type of crystal is useful in identifying it, and thus detection of linear birefringence is especially useful in geology and mineralogy. Linearly polarized light generally has its polarization state altered upon transmission through such a crystal, making it stand out when viewed in between two crossed polarizers, as seen in the photograph, above.
Likewise, in chemistry, rotation of polarization axes in a liquid solution can be a useful measurement. In a liquid, linear birefringence is impossible, however there may be circular birefringence when a chiral molecule is in solution.
When the right and left handed enantiomers of such a molecule are present in equal numbers a so-called racemic mixture then their effects cancel out.
However, when there is only one or a preponderance of one , as is more often the case for organic molecules , a net circular birefringence or optical activity is observed, revealing the magnitude of that imbalance or the concentration of the molecule itself, when it can be assumed that only one enantiomer is present. This is measured using a polarimeter in which polarized light is passed through a tube of the liquid, at the end of which is another polarizer which is rotated in order to null the transmission of light through it.
In many areas of astronomy , the study of polarized electromagnetic radiation from outer space is of great importance. Although not usually a factor in the thermal radiation of stars , polarization is also present in radiation from coherent astronomical sources e. Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field via Faraday rotation.
It has been suggested that astronomical sources caused the chirality of biological molecules on Earth. Unpolarized light, after being reflected by a specular shiny surface, generally obtains a degree of polarization. Polarizing sunglasses exploit this effect to reduce glare from reflections by horizontal surfaces, notably the road ahead viewed at a grazing angle.
Wearers of polarized sunglasses will occasionally observe inadvertent polarization effects such as color-dependent birefringent effects, for example in toughened glass e. The polarized light from LCD monitors see below is very conspicuous when these are worn. Polarization is observed in the light of the sky , as this is due to sunlight scattered by aerosols as it passes through the earth's atmosphere. The scattered light produces the brightness and color in clear skies.
This partial polarization of scattered light can be used to darken the sky in photographs, increasing the contrast. Polarizing filters use these effects to optimize the results of photographing scenes in which reflection or scattering by the sky is involved. Sky polarization has been used for orientation in navigation. The Pfund sky compass was used in the s when navigating near the poles of the Earth's magnetic field when neither the sun nor stars were visible e.
It has been suggested, controversially, that the Vikings exploited a similar device the " sunstone " in their extensive expeditions across the North Atlantic in the 9th—11th centuries, before the arrival of the magnetic compass from Asia to Europe in the 12th century.
Related to the sky compass is the " polar clock ", invented by Charles Wheatstone in the late 19th century. The principle of liquid-crystal display LCD technology relies on the rotation of the axis of linear polarization by the liquid crystal array.
Light from the backlight or the back reflective layer, in devices not including or requiring a backlight first passes through a linear polarizing sheet. That polarized light passes through the actual liquid crystal layer which may be organized in pixels for a TV or computer monitor or in another format such as a seven-segment display or one with custom symbols for a particular product.
The liquid crystal layer is produced with a consistent right or left handed chirality, essentially consisting of tiny helices. This causes circular birefringence, and is engineered so that there is a 90 degree rotation of the linear polarization state. However, when a voltage is applied across a cell, the molecules straighten out, lessening or totally losing the circular birefringence.
On the viewing side of the display is another linear polarizing sheet, usually oriented at 90 degrees from the one behind the active layer. Therefore, when the circular birefringence is removed by the application of a sufficient voltage, the polarization of the transmitted light remains at right angles to the front polarizer, and the pixel appears dark.
With no voltage, however, the 90 degree rotation of the polarization causes it to exactly match the axis of the front polarizer, allowing the light through. Intermediate voltages create intermediate rotation of the polarization axis and the pixel has an intermediate intensity. Displays based on this principle are widespread, and now are used in the vast majority of televisions, computer monitors and video projectors, rendering the previous CRT technology essentially obsolete.
The use of polarization in the operation of LCD displays is immediately apparent to someone wearing polarized sunglasses, often making the display unreadable.
In a totally different sense, polarization encoding has become the leading but not sole method for delivering separate images to the left and right eye in stereoscopic displays used for 3D movies.
This involves separate images intended for each eye either projected from two different projectors with orthogonally oriented polarizing filters or, more typically, from a single projector with time multiplexed polarization a fast alternating polarization device for successive frames. Polarized 3D glasses with suitable polarizing filters ensure that each eye receives only the intended image.
Historically such systems used linear polarization encoding because it was inexpensive and offered good separation. However circular polarization makes separation of the two images insensitive to tilting of the head, and is widely used in 3-D movie exhibition today, such as the system from RealD.
Projecting such images requires screens that maintain the polarization of the projected light when viewed in reflection such as silver screens ; a normal diffuse white projection screen causes depolarization of the projected images, making it unsuitable for this application.
Although now obsolete, CRT computer displays suffered from reflection by the glass envelope, causing glare from room lights and consequently poor contrast. Several anti-reflection solutions were employed to ameliorate this problem. One solution utilized the principle of reflection of circularly polarized light. A circular polarizing filter in front of the screen allows for the transmission of say only right circularly polarized room light. With the right circular polarization filter placed in front of the reflecting glass, the unwanted light reflected from the glass will thus be in very polarization state that is blocked by that filter, eliminating the reflection problem.
The reversal of circular polarization on reflection and elimination of reflections in this manner can be easily observed by looking in a mirror while wearing 3-D movie glasses which employ left- and right-handed circular polarization in the two lenses.
Closing one eye, the other eye will see a reflection in which it cannot see itself; that lens appears black. However the other lens of the closed eye will have the correct circular polarization allowing the closed eye to be easily seen by the open one. All radio and microwave antennas used for transmitting or receiving are intrinsically polarized.
They transmit in or receive signals from a particular polarization, being totally insensitive to the opposite polarization; in certain cases that polarization is a function of direction. Most antennas are nominally linearly polarized, but elliptical and circular polarization is a possibility. As is the convention in optics, the "polarization" of a radio wave is understood to refer to the polarization of its electric field, with the magnetic field being at a 90 degree rotation with respect to it for a linearly polarized wave.
The vast majority of antennas are linearly polarized. In fact it can be shown from considerations of symmetry that an antenna that lies entirely in a plane which also includes the observer, can only have its polarization in the direction of that plane. This applies to many cases, allowing one to easily infer such an antenna's polarization at an intended direction of propagation. So a typical rooftop Yagi or log-periodic antenna with horizontal conductors, as viewed from a second station toward the horizon, is necessarily horizontally polarized.
But a vertical " whip antenna " or AM broadcast tower used as an antenna element again, for observers horizontally displaced from it will transmit in the vertical polarization. A turnstile antenna with its four arms in the horizontal plane, likewise transmits horizontally polarized radiation toward the horizon.
However, when that same turnstile antenna is used in the "axial mode" upwards, for the same horizontally-oriented structure its radiation is circularly polarized. At intermediate elevations it is elliptically polarized.
Polarization is important in radio communications because, for instance, if one attempts to use a horizontally polarized antenna to receive a vertically polarized transmission, the signal strength will be substantially reduced or under very controlled conditions, reduced to nothing. This principle is used in satellite television in order to double the channel capacity over a fixed frequency band. The same frequency channel can be used for two signals broadcast in opposite polarizations.
By adjusting the receiving antenna for one or the other polarization, either signal can be selected without interference from the other. Especially due to the presence of the ground , there are some differences in propagation and also in reflections responsible for TV ghosting between horizontal and vertical polarizations.
AM and FM broadcast radio usually use vertical polarization, while television uses horizontal polarization. At low frequencies especially, horizontal polarization is avoided. That is because the phase of a horizontally polarized wave is reversed upon reflection by the ground. A distant station in the horizontal direction will receive both the direct and reflected wave, which thus tend to cancel each other. This problem is avoided with vertical polarization. Polarization is also important in the transmission of radar pulses and reception of radar reflections by the same or a different antenna.
For instance, back scattering of radar pulses by rain drops can be avoided by using circular polarization. Just as specular reflection of circularly polarized light reverses the handedness of the polarization, as discussed above, the same principle applies to scattering by objects much smaller than a wavelength such as rain drops.
On the other hand, reflection of that wave by an irregular metal object such as an airplane will typically introduce a change in polarization and partial reception of the return wave by the same antenna. There are specific directions for the oscillations of the electric and magnetic fields. This is not the same type of polarization as that discussed for the separation of charges. Waves having such a direction are said to be polarized.
For an EM wave, we define the direction of polarization to be the direction parallel to the electric field. Thus we can think of the electric field arrows as showing the direction of polarization, as in [link].
To examine this further, consider the transverse waves in the ropes shown in [link]. The oscillations in one rope are in a vertical plane and are said to be vertically polarized.
Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves.
For EM waves, the direction of the electric field is analogous to the disturbances on the ropes. The Sun and many other light sources produce waves that are randomly polarized see [link]. Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization. Polaroid materials, invented by the founder of Polaroid Corporation, Edwin Land, act as a polarizing slit for light, allowing only polarization in one direction to pass through.
Polarizing filters are composed of long molecules aligned in one direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave see [link]. A polarizing filter has a polarization axis that acts as a slit passing through electric fields parallel to its direction.
The direction of polarization of an EM wave is defined to be the direction of its electric field. The first filter polarizes the light along its axis.
When the axes of the first and second filters are aligned parallel , then all of the polarized light passed by the first filter is also passed by the second. When the axes are perpendicular, no light is passed by the second.
Only the component of the EM wave parallel to the axis of a filter is passed. Let us call the angle between the direction of polarization and the axis of a filter. If the electric field has an amplitude , then the transmitted part of the wave has an amplitude see [link]. Since the intensity of a wave is proportional to its amplitude squared, the intensity of the transmitted wave is related to the incident wave by.
A polarizing filter transmits only the component of the wave parallel to its axis, , reducing the intensity of any light not polarized parallel to its axis.
Calculating Intensity Reduction by a Polarizing Filter What angle is needed between the direction of polarized light and the axis of a polarizing filter to reduce its intensity by? When the intensity is reduced by , it is or 0. That is,. Using this information, the equation can be used to solve for the needed angle. Solving the equation for and substituting with the relationship between and gives. Solving for yields. A fairly large angle between the direction of polarization and the filter axis is needed to reduce the intensity to of its original value.
This seems reasonable based on experimenting with polarizing films. It is interesting that, at an angle of , the intensity is reduced to. Polarization by Reflection By now you can probably guess that Polaroid sunglasses cut the glare in reflected light because that light is polarized.
You can check this for yourself by holding Polaroid sunglasses in front of you and rotating them while looking at light reflected from water or glass. As you rotate the sunglasses, you will notice the light gets bright and dim, but not completely black.
This implies the reflected light is partially polarized and cannot be completely blocked by a polarizing filter. Vertically polarized light is preferentially refracted at the surface, so that the reflected light is left more horizontally polarized.
The reasons for this phenomenon are beyond the scope of this text, but a convenient mnemonic for remembering this is to imagine the polarization direction to be like an arrow.
Vertical polarization would be like an arrow perpendicular to the surface and would be more likely to stick and not be reflected. Horizontal polarization is like an arrow bouncing on its side and would be more likely to be reflected. Sunglasses with vertical axes would then block more reflected light than unpolarized light from other sources.
Since the part of the light that is not reflected is refracted, the amount of polarization depends on the indices of refraction of the media involved. It can be shown that reflected light is completely polarized at a angle of reflection , given by.
Polarizing filters have a polarization axis that acts as a slit. This slit passes electromagnetic waves often visible light that have an electric field parallel to the axis. This is accomplished with long molecules aligned perpendicular to the axis as shown in [link].
An electromagnetic wave is composed of oscillating electric and magnetic fields. The electric field is strong compared with the magnetic field and is more effective in exerting force on charges in the molecules.
The most affected charged particles are the electrons in the molecules, since electron masses are small. If the electron is forced to oscillate, it can absorb energy from the EM wave. This reduces the fields in the wave and, hence, reduces its intensity. In long molecules, electrons can more easily oscillate parallel to the molecule than in the perpendicular direction.
The electrons are bound to the molecule and are more restricted in their movement perpendicular to the molecule. Thus, the electrons can absorb EM waves that have a component of their electric field parallel to the molecule. The electrons are much less responsive to electric fields perpendicular to the molecule and will allow those fields to pass.
Thus the axis of the polarizing filter is perpendicular to the length of the molecule. Calculating Polarization by Reflection a At what angle will light traveling in air be completely polarized horizontally when reflected from water? All we need to solve these problems are the indices of refraction. Air has water has and crown glass has. The equation can be directly applied to find in each case. Solving for the angle yields. Light reflected at these angles could be completely blocked by a good polarizing filter held with its axis vertical.
Light not reflected is refracted into these media. It will not be completely polarized vertically, because only a small fraction of the incident light is reflected, and so a significant amount of horizontally polarized light is refracted. If you hold your Polaroid sunglasses in front of you and rotate them while looking at blue sky, you will see the sky get bright and dim.
This is a clear indication that light scattered by air is partially polarized. Since light is a transverse EM wave, it vibrates the electrons of air molecules perpendicular to the direction it is traveling. The electrons then radiate like small antennae.Iceland Spar, a rather rare form of the mineral calcite, refracts incident light into two different paths.
The component of the electric field in an EM wave perpendicular to these molecules passes through the filter, while the component parallel to the molecules is absorbed. How much longer will it take the light to deposit a given amount of energy in your eye compared with a single pair of sunglasses? Most sources of light are classified as incoherent and unpolarized or only "partially polarized" because they consist of a random mixture of waves having different spatial characteristics, frequencies wavelengths , phases, and polarization states.
A picket-fence analogy is often used to explain how this dual-filter demonstration works.
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