What is polarization in physics

This is the case for the two hydrogen-oxygen bonds in the water molecule. Electrons shared by these two atoms are drawn more towards the oxygen atom than towards the hydrogen atom. Subsequently, there is a separation of charge, with oxygen having a partially negative charge and hydrogen having a partially positive charge. It is very common to observe this polarization within molecules.

In molecules that have long chains of atoms bonded together, there are often several locations along the chain or near the ends of the chain that have polar bonds. This polarization leaves the molecule with areas that have a concentration of positive charges and other areas with a concentration of negative charges.

This principle is utilized in the manufacture of certain commercial products that are used to reduce static cling. The centers of positive and negative charge within the product are drawn to excess charge residing on the clothes. There is a neutralization of the static charge buildup on the clothes, thus reducing their tendency to be attracted to each other. Other products actually use a different principle.

During manufacturing, a thin sheet is soaked in a solution containing positively charged ions. The sheet is tossed into the dryer with the clothes. Being saturated with positive charges, the sheet is capable of attracting excess electrons that are scuffed off of clothes during the drying cycle.

A complete discussion of the world of atoms, molecules and chemical bonds is beyond the scope of The Physics Classroom. Nonetheless, a model of the atom as a distortable cloud of negative electrons surrounding a positive nucleus becomes essential to understanding how an insulating material can be polarized.

If a charged object is brought near an insulator, the charges on that object are capable of distorting the electron clouds of the insulator atoms.

There is a polarization of the neutral atoms. As shown in the diagrams below, the neutral atoms of the insulator will orient themselves in such a manner as to place the more attractive charge nearest the charged object. Once polarized in this manner, opposites can now attract. A common demonstration performed in class involved bringing a negatively charged balloon near a wooden door or wooden cabinet. The molecules of wood will reorient themselves in such a way as to place their positive charges towards the negatively charged balloon.

The distortion of their electron clouds will result in an alignment of the wood molecules in a manner that makes the wooden cabinet attracted to the negatively charged balloon. In human terms, one might say that the wood does some quick grooming and then places its most attractive side towards the balloon and its most repulsive side away from the balloon. In the world of static electricity, closeness counts. The negative balloon is closer to the positive portion of the wood molecules and further from the more repulsive negative portion.

The balloon and the wall attract with sufficient force to cause the balloon to stick to the wall. From a mechanics standpoint, we would say that the balloon and the wall are pressed together with a large force.

The large normal force on the balloon results in a large static friction force. This friction force balances the downward force of gravity and the balloon remains at rest. Another common physics and chemistry demonstration involves using a charged object to deflect a stream of water from its path.

Most often, a comb is charged negatively by combing one's hair or a rubber balloon is charged in a similar manner. The negatively charged object is then brought near to a falling stream of water, causing the stream to be attracted to the comb or balloon and alter its direction of fall. The demonstration illustrates the polar nature of water molecules. The hydrogen atoms serve as the positive poles within a water molecule; oxygen serves as the negative pole.

Molecules of a liquid are free to rotate and move about; the water molecules realign themselves in order to put their positive poles towards the negatively charged object. Once polarized, the stream and the balloon or comb are attracted. As the water molecules within the stream fall past the balloon, this realignment of individual molecules happens quickly and the entire stream is deflected from its original downward direction. Examples of the attraction between charged objects and neutral objects are numerous and often demonstrated by physics teachers.

Paper bits become polarized and are attracted to a charged piece of acetate. Small penguins cut from a sheet of paper are attracted to a charged plastic golf tube and demonstrate their happy feet. A long wooden 2x4 is placed on a pivot and becomes polarized and attracted to a charged golf tube. To the astonishment of students, the force of attraction on the wood is large enough to rotate it about the pivot point. Perhaps the biggest misconception that pertains to polarization is the belief that polarization involves the charging of an object.

Polarization is not charging! When an object becomes polarized, there is simply a redistribution of the centers of positive and negative charges within the object. Either by the movement of electrons across the surface of the object as is the case in conductors or through the distortion of electron clouds as is the case in insulators , the centers of positive and negative charges become separated from each other.

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. Sunsets are viewed with light traveling straight from the Sun toward us.

When blue light is scattered out of this path, the remaining red light dominates the overall appearance of the setting Sun. 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. What direction would you expect the polarization to have and would you expect it to be?

If you lie on a beach looking at the water with your head tipped slightly sideways, your polarized sunglasses do not work very well. Why not? The axis of polarization for the sunglasses has been rotated. What angle is needed between the direction of polarized light and the axis of a polarizing filter to cut its intensity in half?

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? Two polarizing sheets and are placed together with their transmission axes oriented at an angle to each other. What is when only of the maximum transmitted light intensity passes through them? Suppose that in the preceding problem the light incident on is unpolarized. At the determined value of , what fraction of the incident light passes through the combination?

What angle would the axis of a polarizing filter need to make with the direction of polarized light of intensity to reduce the intensity to? At the end of Figure , it was stated that the intensity of polarized light is reduced to of its original value by passing through a polarizing filter with its axis at an angle of to the direction of polarization. Verify this statement.

Show that if you have three polarizing filters, with the second at an angle of to the first and the third at an angle of to the first, the intensity of light passed by the first will be reduced to of its value.

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. Three polarizing sheets are placed together such that the transmission axis of the second sheet is oriented at to the axis of the first, whereas the transmission axis of the third sheet is oriented at in the same sense to the axis of the first.

What fraction of the intensity of an incident unpolarized beam is transmitted by the combination? What is the refractive index of the plastic? Use this time to calculate the speed of light. The distance from the wheel to the mirror was Assuming he measured the speed of light accurately, what was the angular velocity of the wheel?

Suppose you have an unknown clear substance immersed in water, and you wish to identify it by finding its index of refraction.

You arrange to have a beam of light enter it at an angle of , and you observe the angle of refraction to be. What is the index of refraction of the substance and its likely identity? Shown below is a ray of light going from air through crown glass into water, such as going into a fish tank. Calculate the amount the ray is displaced by the glass given that the incident angle is and the glass is 1. Considering the previous problem, show that is the same as it would be if the second medium were not present.

At what angle is light inside crown glass completely polarized when reflected from water, as in a fish tank? Light reflected at from a window is completely polarized. Can the gem be a diamond? The gem is not a diamond it is zircon. Unreasonable results Suppose light travels from water to another substance, with an angle of incidence of and an angle of refraction of. We cannot have , since this would imply a speed greater than c. The refracted angle is too big relative to the angle of incidence.

Unreasonable results Light traveling from water to a gemstone strikes the surface at an angle of and has an angle of refraction of. Suppose you put on two pairs of polarizing 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. Assuming the sunlight is unpolarized and the polarizers are efficient, what is the initial rate of heating of the water in , assuming it is.

The aluminum beaker has a mass of Light shows staged with lasers use moving mirrors to swing beams and create colorful effects. Show that a light ray reflected from a mirror changes direction by when the mirror is rotated by an angle. Taking the boundary between nearly empty space and the atmosphere to be sudden, calculate the angle of refraction for sunlight.

This lengthens the time the Sun appears to be above the horizon, both at sunrise and sunset. Now construct a problem in which you determine the angle of refraction for different models of the atmosphere, such as various layers of varying density.

Your instructor may wish to guide you on the level of complexity to consider and on how the index of refraction varies with air density.

First part:. The remainder depends on the complexity of the solution the reader constructs. A light ray entering an optical fiber surrounded by air is first refracted and then reflected as shown below. Show that if the fiber is made from crown glass, any incident ray will be totally internally reflected. A light ray falls on the left face of a prism see below at the angle of incidence for which the emerging beam has an angle of refraction at the right face. Show that the index of refraction n of the glass prism is given by.

If and the base angles of the prism are each what is n? If the apex angle in the previous problem is and , what is the value of? The light incident on polarizing sheet is linearly polarized at an angle of with respect to the transmission axis of. Sheet is placed so that its axis is parallel to the polarization axis of the incident light, that is, also at with respect to. 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 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 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 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. Polarization has a wealth of other applications besides their use in glare-reducing sunglasses.

We say that the charge in the can has been polarized. In general terms, polarization means to separate into opposites. In the political world, we often observe that a collection of people becomes polarized over some issue.

For instance, we might say that the United States has become polarized over the issue of the death penalty. That is, the citizens of the United States have been separated into opposites - those who are for the death penalty and those who are against the death penalty. In the context of electricity, polarization is the process of separating opposite charges within an object.

The positive charge becomes separated from the negative charge. By inducing the movement of electrons within an object, one side of the object is left with an excess of positive charge and the other side of the object is left with an excess of negative charge. Charge becomes separated into opposites. The polarization process always involves the use of a charged object to induce electron movement or electron rearrangement.

In the above diagram and accompanying discussion, electrons within a conducting object were induced into moving from the left side of the conducting can to the right side of the can. Being a conductor, electrons were capable of moving from atom to atom across the entire surface of the conductor. But what if the object being polarized is an insulator? Electrons are not free to move across the surface of an insulator. How can an insulator such as a wooden wall be polarized? Polarization can occur within insulators, but the process occurs in a different manner than it does within a conductor.

In a conducting object, electrons are induced into movement across the surface of the conductor from one side of the object to the opposite side. In an insulator, electrons merely redistribute themselves within the atom or molecules nearest the outer surface of the object.

To understand the electron redistribution process, it is important to take another brief excursion into the world of atoms, molecules and chemical bonds. The electrons surrounding the nucleus of an atom are believed to be located in regions of space with specific shapes and sizes.

The actual size and shape of these regions is determined by the high-powered mathematical equations common to Quantum Mechanics. Rather than being located a specific distance from the nucleus in a fixed orbit, the electrons are simply thought of as being located in regions often referred to as electron clouds. At any given moment, the electron is likely to be found at some location within the cloud.

The electron clouds have varying density; the density of the cloud is considered to be greatest in the portion of the cloud where the electron has the greatest probability of being found at any given moment. And conversely, the electron cloud density is least in the regions where the electron is least likely to be found. In addition to having varying density, these electron clouds are also highly distortable.

The presence of neighboring atoms with high electron affinity can distort the electron clouds around atoms. Rather than being located symmetrically about the positive nucleus, the cloud becomes asymmetrically shaped. As such, there is a polarization of the atom as the centers of positive and negative charge are no longer located in the same location. The atom is still a neutral atom; it has just become polarized.

The discussion becomes even more complex and perhaps too complex for our purposes when we consider molecules - combination of atoms bonded together. In molecules, atoms are bonded together as protons in one atom attract the electrons in the clouds of another atom.

This electrostatic attraction results in a bond between the two atoms. Electrons are shared by the two atoms as they begin to overlap their electron clouds. If the atoms are of different types for instance, one atom is Hydrogen and the other atom is Oxygen , then the electrons within the clouds of the two atoms are not equally shared by the atoms.

The clouds become distorted, with the electrons having the greatest probability of being found closest to the more electron-greedy atom. The bond is said to be a polar bond. The distribution of electrons within the cloud is shifted more towards one atom than towards the other atom. This is the case for the two hydrogen-oxygen bonds in the water molecule. Electrons shared by these two atoms are drawn more towards the oxygen atom than towards the hydrogen atom.

Subsequently, there is a separation of charge, with oxygen having a partially negative charge and hydrogen having a partially positive charge. It is very common to observe this polarization within molecules.