In addition to depending on the type of substance, the amount and direction of rotation depends on a number of factors. Among these is the concentration of the substance, the distance the light travels through it, and the wavelength of light. Optical activity is due to the asymmetric shape of molecules in the substance, such as being helical.
Measurements of the rotation of polarized light passing through substances can thus be used to measure concentrations, a standard technique for sugars. It can also give information on the shapes of molecules, such as proteins, and factors that affect their shapes, such as temperature and pH. Optical activity is the ability of some substances to rotate the plane of polarization of light passing through them.
The rotation is detected with a polarizing filter or analyzer. Glass and plastic become optically active when stressed; the greater the stress, the greater the effect. Optical stress analysis on complicated shapes can be performed by making plastic models of them and observing them through crossed filters, as seen in Figure It is apparent that the effect depends on wavelength as well as stress.
The wavelength dependence is sometimes also used for artistic purposes. Optical stress analysis of a plastic lens placed between crossed polarizers. Another interesting phenomenon associated with polarized light is the ability of some crystals to split an unpolarized beam of light into two.
Such crystals are said to be birefringent see Figure Each of the separated rays has a specific polarization. Birefringent crystals can be used to produce polarized beams from unpolarized light. Some birefringent materials preferentially absorb one of the polarizations.
These materials are called dichroic and can produce polarization by this preferential absorption. This is fundamentally how polarizing filters and other polarizers work. The interested reader is invited to further pursue the numerous properties of materials related to polarization. Birefringent materials, such as the common mineral calcite, split unpolarized beams of light into two.
Skip to main content. Wave Optics. Search for:. Polarization Learning Objectives By the end of this section, you will be able Discuss the meaning of polarization. Discuss the property of optical activity of certain materials. Example 1. 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 Strategy When the intensity is reduced by Discussion A fairly large angle between the direction of polarization and the filter axis is needed to reduce the intensity to Example 2.
Calculating Polarization by Reflection At what angle will light traveling in air be completely polarized horizontally when reflected from water? From glass? Strategy All we need to solve these problems are the indices of refraction. Discussion Light reflected at these angles could be completely blocked by a good polarizing filter held with its axis vertical.
Take-Home Experiment: Polarization Find Polaroid sunglasses and rotate one while holding the other still and look at different surfaces and objects. Conceptual Questions Under what circumstances is the phase of light changed by reflection?
Is the phase related to polarization? Can a sound wave in air be polarized? No light passes through two perfect polarizing filters with perpendicular axes. However, if a third polarizing filter is placed between the original two, some light can pass. Why is this? Under what circumstances does most of the light pass? Explain what happens to the energy carried by light that it is dimmed by passing it through two crossed polarizing filters.
How does this relate to the fact that the sky is blue? Using the information given in the preceding question, explain why sunsets are red. Part of the light will be refracted into the surface. Describe how you would do an experiment to determine the polarization of the refracted light.
The angle between the axes of two polarizing filters is By how much does the second filter reduce the intensity of the light coming through the first? What angle would the axis of a polarizing filter need to make with the direction of polarized light of intensity 1.
At the end of Example 1, it was stated that the intensity of polarized light is reduced to Verify this statement. This is in contrast to having only the first and third, which reduces the intensity to zero, so that placing the second between them increases the intensity of the transmitted light.
Hint: Use the trigonometric identities cos At what angle will light reflected from diamond be completely polarized? At what angle will this light be completely polarized? At what angle is light inside crown glass completely polarized when reflected from water, as in a fish tank?
Light reflected at Can the gem be a diamond? Integrated Concepts. If a polarizing filter reduces the intensity of polarized light to Suppose you put on two pairs of Polaroid sunglasses with their axes at an angle of 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? Assume the lenses are clear except for their polarizing characteristics.
Two polarizing sheets of plastic are placed in front of the lens with their axes at an angle of The aluminum beaker has a mass of Licenses and Attributions. Electrons in these media can travel more easily along the direction of the atomic chains. This allows the electric fields which are oriented in the direction of the atomic chains to transfer their energy to the electrons in the medium. The component of the electric field which is perpendicular to the atomic chains cannot give energy to the electrons, because the electrons cannot move in that direction.
This means the wave component aligned with the atomic chains is absorbed, while the wave component nonaligned is transmitted. If we are given any polarization of light we can break it into components: a component for which the polarization of the wave is in the parallel to the chains of molecules which will be absorbed by the electrons and a component which is perpendicular to the chains of molecules which will pass through as it cannot be absorbed.
We call the parallel axis the absorption axis , because this component of the light is absorbed. We call this perpendicular direction the transmission axis or polarizer axis , because light that passes through the polarizer is polarized in this direction. The image above is a diagram of a polarizer. Because the electrons cannot oscillate in the direction of the polarizer axis, any light that passes through this polarizer will come out polarized in that direction horizontally, in this diagram.
Light polarized along a vertical axis, traveling into the page, hits a polarizer. What is the intensity of the light coming out of the polarizer as a fraction of the intensity of incoming light? Which way is the light polarized? The polarization axis and the electric field are shown on the picture above. First we break the electric field into a part that is along the polarizer axis which will be transmitted and a part that is along the atomic chains of the material which will be absorbed.
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:. 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.
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. 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.
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