Transcript File
Chapter 9.6 Polarization Simple concepts of Polarization • Waves on a string • Diagram of vertical wave on a string (string is always in the vertical plane) • Diagram of horizontal wave on a string (string is always in the horizontal plane) • String waves are plane polarized because the string is always in a fixed plane. Light • Light is a transverse wave in which an electric field and a magnetic field at right angles to each other propagate along a direction that is normal to both fields. The oscillations are perpendicular to the direction of travel. Longitudinal waves cannot be polarized. • Light that is vibrating in more than one plane is referred to as unpolarized light. • We focus only on the electric field of the electromagnetic wave for polarization of light. • An electromagnetic wave is plane polarized if: 1) Electric field always lies in the same plane 2) Unpolarized light becomes polarized by using a sheet of material with a molecular structure that only allows a specific orientation of the electric field to go through • Diagram of polarized light • • Example of polarizer: 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. 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. Two Polaroids Picket Fence Analogy Polarization by Reflection • Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. • Natural light that is incident on calm water will enter the water and be refracted and some will be reflected. • Equations and Diagram • Reflected ray is always perpendicular to refracted ray. The reflected ray will be completely plane polarized. • Equations • 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. Polarization by Reflection Polarization by Refraction • 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. • Brewster’s law- if a ray of light is incident on the surface of a transparent medium so that the reflected ray is perpendicular to the refracted ray, the reflected ray will be plane polarized. • Brewster’s law equation • If there is a reflected glare off the surface of a lake, it is partially polarized light that was incident at an angle too large to be refracted. The glare prevents us from seeing anything below the surface of the water. Polaroid sunglasses help reduce glare. • Diagram of light hitting lake Polaroid Combinations • Two polaroids (polarizing materials) are mutually perpendicular so no light passes through the combination. Light passes through the first polaroid and plane polarized light is incident on the second polaroid. Orientation of the second polaroid is at 90 degrees so no light is transmitted. • Diagram of polaroid combinations • First polaroid is called the polarizer. • Second polaroid is called the analyzer. • The analyzer determines both whether the light is polarized and the plane of polarization. • If the analyzer is not perpendicular to the polarizer but oriented at angle theta, intensity of transmitted light is given by Malus’ Law • Diagram of Malus’ Law • Equation of Malus’ Law Optically Active Substances • Optically active- a material that rotates the plane of incident plane polarized light (either clockwise or counterclockwise direction) • Examples: quartz and sugars (glucose and fructose) • The amount of optical activity depends on the concentration of solution. • How to use polarization to determine the concentration of solutions. • Determine optical activity of fructose by using polarized light. Different angles of rotation produce colors. • Transparent materials can become optically active under stress- polarization used in stress analysis. Place stressed material between the two crossed polaroids and the colors become visible. Practical Application • Engineers can make small scale models of structures made of transparent material (bridges, etc) and subject it to stresses anticipated in real life. Then they analyze it between two crossed polaroids to view how structure held up during stress. The analysis of colored patterns can help modify and improve design. • Stress can make a substance optically active and colors can be seen when polarized light passes through them. Liquid-crystal displays (LCDs) • LCDs made of arrays of pixels between two crossed polaroids. Voltage is applied, electric field moves liquid crystal molecules in the pixels. • Example: liquid crystals twist and untwist according to strength of field reflector at the back of the display and when no voltage is applied, light reflects straight back. When display is switched on, liquid crystal molecules untwist and block light.