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Light Waves and Color - Lesson 1

How Do We Know Light Behaves as a Wave?

Wavelike Behaviors of Light

An age-old debate that has persisted among scientists is related to the question, "Is light a wave ora stream of particles?" Very noteworthy and distinguished physicists have taken up each side of the

argument, providing a wealth of evidence for each side. The fact is that light exhibits behaviors thatare characteristic of both waves and particles. In this unit ofThe Physics Classroom Tutorial, thefocus will be on the wavelike nature of light.

Light exhibits certain behaviors that are characteristic of any wave and would be difficult to explainwith a purely particle-view. Light reflects in the same manner that any wave would reflect. Lightrefracts in the same manner that any wave would refract. Light diffracts in the same manner that

any wave would diffract. Light undergoes interference in the same manner that any wave wouldinterfere. And light exhibits the Doppler effect just as any wave would exhibit the Doppler effect.Light behaves in a way that is consistent with our conceptual and mathematical understanding ofwaves. Since light behaves like a wave, one would have good reason to believe that it might be a

wave. In Lesson 1, we will investigate the variety of behaviors, properties and characteristics oflight that seem to support the wave model of light. On this page, we will focus on three specificbehaviors - reflection, refraction and diffraction.

A wave doesn't just stop when it reaches the end of the medium. Rather, a wave will undergo

certain behaviors when it encounters the end of the medium. Specifically, there will be somereflection off the boundary and some transmission into the new medium. The transmitted wave

undergoes refraction (or bending) if it approaches the boundary at an angle. If the boundary ismerely an obstacle implanted within the medium, and if the dimensions of the obstacle are smaller

than the wavelength of the wave, then there will be very noticeable diffraction of the wave aroundthe object. Each one of these behaviors - reflection, refraction and diffraction - is characterized byspecific conceptual principles and mathematical equations. The reflection, refraction, and diffractionof waves were first introduced in Unit 10 of The Physics Classroom Tutorial. In Unit 11 of The

Physics Classroom Tutorial, the reflection, refraction, and diffraction of sound waves was discussed.Now we will see how light waves demonstrate their wave nature by reflection, refraction anddiffraction.

Reflection of Light Waves

All waves are known to undergo reflection or the bouncing off of an obstacle. Most people are very

accustomed to the fact that light waves also undergo reflection. The reflection of light waves off of amirrored surface results in the formation of an image. One characteristic

of wave reflection is that the angle at which the wave approaches a flatreflecting surface is equal to the angle at which the wave leaves the

surface. This characteristic is observed for water waves and soundwaves. It is also observed for light waves. Light, like any wave, follows

the law of reflection when bouncing off surfaces. The reflection of lightwaves will be discussed in more detail in Unit 13 of The PhysicsClassroom. For now, it is enough to say that the reflective behavior of

light provides evidence for the wavelike nature of light.

Refraction of Light Waves

All waves are known to undergo refraction when they pass from onemedium to another medium. That is, when a wavefront crosses theboundary between two media, the direction that the wavefront is moving

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undergoes a sudden change; the path is "bent." This behavior of wave refraction can be describedby both conceptual and mathematical principles. First, the direction of "bending" is dependent upon

the relative speed of the two media. A wave will bend one way when it passes from a medium inwhich it travels slowly into a medium in which it travels fast; and if moving from a fast medium to a

slow medium, the wavefront will bend in the opposite direction. Second, the amount of bending isdependent upon the actual speeds of the two media on each side of the boundary. The amount ofbending is a measurable behavior that follows distinct mathematical equations. These equations are

based upon the speeds of the wave in the two media and the angles at which the wave approachesand departs from the boundary. Light, like any wave, is known to refract as it passes from onemedium into another medium. In fact, a study of the refraction of light reveals that its refractivebehavior follows the same conceptual and mathematical rules that govern the refractive behavior ofother waves such as water waves and sound waves. The refraction of light waves will be discussed

in more detail in Unit 14 of The Physics Classroom Tutorial. For now, it is enough to say that therefractive behavior of light provides evidence for the wavelike nature of light.

Diffraction of Light Waves

Reflection involves a change in direction of waves when they bounce offa barrier. Refraction of waves involves a change in the direction of waves

as they pass from one medium to another. And diffraction involves achange in direction of waves as they pass through an opening or around

an obstacle in their path. Water waves have the ability to travel aroundcorners, around obstacles and through openings. Sound waves do thesame. But what about light? Do light waves bend around obstacles andthrough openings? If they do, then it would provide still more evidence

to support the belief that light behaves as a wave.

When light encounters an obstacle in its path, the obstacle blocks the

light and tends to cause the formation of a shadow in the region behindthe obstacle. Light does not exhibit a very noticeable ability to bendaround the obstacle and fill in the region behind it with light.Nonetheless, light does diffract around obstacles. In fact, if you observe

a shadow carefully, you will notice that its edges are extremely fuzzy. Interference effects occur due

to the diffraction of light around different sides of the object, causing the shadow of the object to befuzzy. This is often demonstrated in a Physics classroom with a laser light and penny demonstration.Light diffracting around the right edge of a penny can constructively and destructively interfere with

light diffracting around the left edge of the penny. The result is that an interference pattern iscreated; the pattern consists of alternating rings of light and darkness. Such a pattern is only

noticeable if a narrow beam of monochromatic light (i.e., single wavelength light) is passed directedat the penny. The photograph at the right shows an interference pattern created in this manner.

Since, light waves are diffracting around the edges of the penny, the waves are broken up intodifferent wavefronts that converge at a point on a screen to produce the interference pattern shown

in the photograph. Can you explain this phenomenon with a strictly particle-view of light? Thisamazing penny diffraction demonstration provides another reason why believing that light has awavelike nature makes cents (I mean "sense"). These interference effects will be discussed in moredetaillater in this lesson.

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Light behaves as a wave - it undergoes reflection, refraction, and diffraction just like any wavewould. Yet there is still more reason to believe in the wavelike nature of light.Continue with Lesson1to learn about more behaviors that could never be explained by a strictly particle-view of light.

Two Point Source Interference

Wave interference is a phenomenon that occurs when two waves meet while traveling along thesame medium. The interference of wavescauses the medium to take on a shape that results fromthe net effect of the two individual waves upon the particles of the medium. Wave interference can

be constructive or destructive in nature. Constructive interference occurs at any location alongthe medium where the two interfering waves have a displacement in the same direction. Forexample, if at a given instant in time and location along the medium, the crest of one wave meetsthe crest of a second wave, they will interfere in such a manner as to produce a "super-crest."

Similarly, the interference of a trough and a trough interfere constructively to produce a "super-trough." Destructive interference occurs at any location along the medium where the twointerfering waves have a displacement in the opposite direction. For example, the interference of acrest with a trough is an example of destructive interference. Destructive interference has the

tendency to decrease the resulting amount of displacement of the medium. Interference principleswere first introduced inUnit 10 of The Physics Classroom Tutorial. The principles were subsequently

applied to the interference of sound waves inUnit 11 of The Physics Classroom Tutorial.

A defining moment in the history of the debate concerning the nature of light occurred in the early

years of the nineteenth century. Thomas Young showed that an interference pattern results whenlight from two sources meets up while traveling through the same medium. To understand Young'sexperiment, it is important to back up a few steps and discuss the interference of water waves thatoriginate from two points.

In Unit 10, the value of a ripple tank in the study of water wavebehavior was introduced and discussed. If an object bobs up and down

in the water, a series water waves in the shape of concentric circles willbe produced within the water. If two objects bob up and down with thesame frequency at two different points, then two sets of concentriccircular waves will be produced on the surface of the water. These

concentric waves will interfere with each other as they travel across thesurface of the water. If you have ever simultaneously tossed twopebbles into a lake (or somehow simultaneously disturbed the lake intwo locations), you undoubtedly noticed the interference of these waves.

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The crest of one wave will interfere constructively with the crest of the second wave to produce alarge upward displacement. And the trough of one wave will interfere constructively with the trough

of the second wave to produce a large downward displacement. And finally the crest of one wavewill interfere destructively with the trough of the second wave to produce no displacement. In a

ripple tank, this constructive and destructive interference can be easily controlled and observed. Itrepresents a basic wave behavior that can be expected of any type of wave.

Two-Point Source Interference Patterns

The interference of two sets of periodic and concentric waves with the same frequency produces aninteresting pattern in a ripple tank. The diagram at the right depicts an interference pattern

produced by two periodic disturbances. The crests are denoted by the thick lines and the troughsare denoted by the thin lines. Thus, constructive interference occurs wherever a thick line meets athick line or a thin line meets a thin line; this type of interference results in the formation of anantinode. The antinodesare denoted by a red dot. Destructive interference occurs wherever a thick

line meets a thin line; this type of interference results in the formation of a node. Thenodes aredenoted by a blue dot. The pattern is a standing wave pattern, characterized by the presence of

nodes and antinodes that are "standing still" - i.e., always located at the same position on themedium. The antinodes (points where the waves always interfere constructively) seem to be located

along lines - creatively called antinodal lines. The nodes also fall along lines - called nodal lines.The two-point source interference pattern is characterized by a pattern of alternating nodal and

antinodal lines. There is a central line in the pattern - the line that bisects the line segment that isdrawn between the two sources is an antinodal line. This central antinodal line is a line of pointswhere the waves from each source always reinforce each other by means of constructiveinterference. The nodal and antinodal lines are included on the diagram below.

A two-point source interference pattern always has an alternating pattern of nodal and antinodallines. There are however some features of the pattern that can be modified. First, a change in

wavelength (or frequency) of the source will alter the number of lines in the pattern and alter theproximity or closeness of the lines. An increase in frequency will result in more lines per centimeterand a smaller distance between each consecutive line. And a decrease in frequency will result infewer lines per centimeter and a greater distance between each consecutive line.

Second, a change in the distance between the two sources will also alter the number of lines and

the proximity or closeness of the lines. When the sources are moved further apart, there are more

lines produced per centimeter and the lines move closer together. These two general cause-effectrelationships apply to any two-point source interference pattern, whether it is due to water waves,sound waves, or any other type of wave.

Changing Source Separation Changing Wavelength

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Two-Point Source Light Interference Patterns

Any type of wave, whether it be a water wave or a sound wave should produce a two-point source

interference pattern if the two sources periodically disturb the medium at the same frequency. Sucha pattern is always characterized by a pattern of alternating nodal and antinodal lines. Of course,the question should arise and indeed did arise in the early nineteenth century: Can light produce atwo-point source interference pattern? If light is found to produce such a pattern, then it will

provide more evidence in support of the wavelike nature of light.

Before we investigate the evidence in detail, let's discuss what one might observe if light were to

undergo two-point source interference. What would happen if a "crest" of one light wave interferedwith a "crest" of a second light wave? And what would happen if a "trough" of one light waveinterfered with a "trough" of a second light wave? And finally, what would happen if a "crest" of onelight wave interfered with a "trough" of a second light wave?

Whenever light constructively interferes (such as when a crest meeting a crest or a trough meetinga trough), the two waves act to reinforce one another and to produce a "super light wave." On the

other hand, whenever light destructively interferes (such as when a crest meets a trough), the twowaves act to destroy each other and produce no light wave. Thus, the two-point source interference

pattern would still consist of an alternating pattern of antinodal lines and nodal lines. However forlight waves, the antinodal lines are equivalent to bright lines and the nodal lines are equivalent to

dark lines. If such an interference pattern could be created by two light sources and projected ontoa screen, then there ought to be an alternating pattern of dark and bright bands on the screen. And

since the central line in such a pattern is an antinodal line, the central band on the screen ought tobe a bright band.

In 1801, Thomas Young successfully showed that light does produce a two-point source interference

pattern. In order to produce such a pattern, monochromatic light must be used. Monochromaticlight is light of a single color; by use of such light, the two sources will vibrate with the samefrequency. It is also important that the two light waves be vibrating in phase with each other; thatis, the crest of one wave must be produced at the same precise time as the crest of the second

wave. (This is often referred to as coherent light.) To accomplish this, Thomas Young used asingle light source and projected the light onto two pinholes. The light from the source will thendiffract through the pinholes and the pattern can be projected onto a screen. Since there is only onesource of light, the set of two waves that emanate from the pinholes will be in phase with each

other. As expected, the use of a monochromatic light source and pinholes to generate in-phase lightwaves resulted in a pattern of alternating bright and dark bands on the screen. A typical appearance

of the pattern is shown below.

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Young's two-point source interference experiment is often performed in a Physics course with laser

light. It is found that the same principles that apply to water waves in a ripple tank also apply tolight waves in the experiment. For instance, a higher frequency light source should produce aninterference pattern with more lines per centimeter in the pattern and a smaller spacing betweenlines. Indeed this is observed to be the case. Furthermore, a greater distance between slits should

produce an interference pattern with more lines per centimeter in the pattern and a smaller spacingbetween lines. Again, this is observed to be the case.

Most astounding of all is that Thomas Young was able to use wave principles to measure thewavelength of light. Details on the development of Young's equation and further information abouthis experiment are provided in Lesson 3 of this unit. For now, the emphasis is on how the same

characteristics observed of water waves in a ripple tank are also observed of light waves. ThomasYoung's findings provide even more evidence for the scientists of the day that light behaves as awave. After all, can a stream of particles do all this?

Thin Film Interference

The emphasis of Lesson 1 of this unit is to present some evidence that has historically supportedthe view that light behaves as a wave. The reflection, refraction and diffraction of light waves is onestrand of evidence. The interference of light waves is a second strand of evidence. In the early

nineteenth century, Thomas Young showedthat the interference of light passing through two slitsproduces an interference pattern when projected on a screen. In this section of Lesson 1, we willinvestigate another example of interference that provides further evidence in support of the

wavelike behavior of light.

Perhaps you have witnessed streaks of color on a car windshield shortly after it has been swiped bya windshield wiper or a squeegee at a gas station. The momentary streaks of color are the result of

interference of light by the very thin film of water or soap that remains on the windshield. Orperhaps you have witnessed streaks of color in a thin film of oil resting upon a water puddle orconcrete driveway. These streaks of color are the result of the interference of light by the very thinfilm of oil that is spread over the water surface. This form of interference is commonly called thin

film interference and provides another line of evidence for the wave behavior of light.

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Light wave interference results when two waves are traveling through a medium and meet upat the same location. So what exactly is causing this thin film interference? What is the source

of the two waves? When a wave (light waves included) reaches the boundary between twomedia, a portion of the wave reflects off the boundary and a portion is transmitted across the

boundary. The reflected portion of the wave remains in the original medium. The transmittedportion of the wave enters the new medium and continues traveling through it until it reachesa subsequent boundary. If the new medium is a thin film, then the transmitted wave does not

travel far before it reaches a new boundary and undergoesthe usual reflection and transmission behavior. Thus, thereare two waves that emerge from the film - one wave that isreflected off the top of the film (wave 1 in the diagram) andthe other wave that reflects off the bottom of the film (wave

2 in the diagram).

These two waves could interfere constructively if they meet

two conditions. One condition is that the two waves must berelatively close together such that their crests and troughs canmeet up with each other and cause the interference. To meet this condition, the light must beincident at angles close to zero with respect to the normal. (This is not shown in the diagram above

in order to space out the waves for clarity sake.) A second condition that must be met is that thewave that travels through the film and back into the original medium must have traveled just the

right distance such that it is in phase with the other reflected wave. Two waves that are in phaseare waves that are always at the same point on their wave cycle. That is, the two waves must be

forming crests at the same location and at the same moment in time and forming troughs at thesame location and at the same moment in time. In order for the second condition to occur, the

thickness of the film must bejust perfect.

If wave 1 and wave 2 meet these two conditions as they reflect and exit the film, then they will

constructively interfere. As will be learned in Lesson 2, light that is visible to our eyes consists of acollection of light waves of varying wavelength. Each wavelength is characterized by its own color.So a red light wave has a different wavelength than an orange light wave that has a differentwavelength than a yellow light wave. While the thickness of a film at a given location may not allow

a red and an orange light wave to emerge from the film in phase, it may bejust perfectto allow ayellow light wave to emerge in phase. So at a given location on the film, the yellow light waveundergoes constructive interference and becomes brighter than the other colors within the incidentlight. As such, the film appears yellow when viewed by incident sunlight. Other locations of the film

may be just perfect to constructively reinforce red light. And still others area of the film may be ofperfect thickness for the constructive reinforcement of green light. Because different locations of thefilm may be of appropriate thickness to reinforce different colors of light, the thin film will showstreaks of color when viewed from above.

While the mathematics of thin film interference can become quite complicated, it is clear from thisdiscussion that thin film interference is another phenomenon that can only be explained using a

wave model of light.

Polarization

A light wave is an electromagnetic wavethat travels through the vacuum of outer space. Light

waves are produced by vibrating electric charges. The nature of such electromagnetic waves isbeyond the scope ofThe Physics Classroom Tutorial. For our purposes, it is sufficient to merely say

that an electromagnetic wave is a transverse wave that has both an electric and a magneticcomponent.

The transverse nature of an electromagnetic wave is quite different from any other type of wavethat has been discussed in The Physics Classroom Tutorial. Let's suppose that we use the customaryslinky to model the behavior of an electromagnetic wave. As an electromagnetic wave traveledtowards you, then you would observe the vibrations of the slinky occurring in more than one plane

of vibration. This is quite different than what you might notice if you were to look along a slinky andobserve a slinky wave traveling towards you. Indeed, the coils of the slinky would be vibrating backand forth as the slinky approached; yet these vibrations would occur in a single plane of space. That

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The alignment of these molecules gives the filter a polarization axis. This polarization axis extendsacross 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 axisare 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 allowthe vertical vibrations to be transmitted (see diagram above). On the other hand, a Polaroid filterwith 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 avariety 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 theobject 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 thesecond filter, an orientation can be found in which all the light from an object is blocked and theobject can no longer be seen when viewed through two filters. What happened? In thisdemonstration, 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 secondfilter since its polarization filter is aligned in a horizontal direction. While you are unable to see theaxes on the filter, you will know when the axes are aligned perpendicular to each other becausewith 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 thatthey 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 vibratesin 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 verticalvibrations 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 bythe second fence. This is depicted in the diagram below.

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In the same manner, two Polaroid filters oriented with their polarization axes perpendicular to eachother will block all the light. Now that's a pretty cool observation that could never be explained by aparticle 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 surfaceand upon the material that the surface is made of. Metallic surfaces reflect light with a variety ofvibrational 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 ofvibrations in a plane parallel to the reflecting surface. A person viewing objects by means of lightreflected 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 allowsfor the blocking of this partially polarized light. By blocking the plane-polarized light, the glare isreduced and the fisherman can more easily see fish located under the water.

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Polarization by Refraction

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 twomaterials, the path of the beam changes its direction. Therefracted beam acquires some degree of polarization. Mostoften, the polarization occurs in a plane perpendicular to the

surface. The polarization of refracted light is oftendemonstrated in a Physics class using a unique crystal thatserves as a double-refracting crystal. Iceland Spar, a ratherrare form of the mineral calcite, refracts incident light into

two different paths. The light is splitinto two beams uponentering the crystal. Subsequently, if an object is viewed bylooking through an Iceland Spar crystal, two images will beseen. The two images are the result of the double refraction

of light. Both refracted light beams are polarized - one in adirection 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 polarizingfilter can be used to completely block one of the images. Ifthe 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 filteris then turned 90-degrees in either direction, the second image reappears and the first imagedisappears. Now that's pretty neat observation that could never be observed if light did not exhibitany wavelike behavior.

Watch It!

In the demonstration below, the word PHUN (as in Physics is ...) is written on the glass panel of aclassroom-style overhead projector. A sample of Iceland spar is placed over the word PHUN. Two

images of the word PHUN can be faintly seen in the early seconds of the movie. The crystal doublerefracts light that passes through it. At about the 8-second mark, a Polaroid filter is placed over the

crystal and rotated. As it rotates, the two images alternately fade in and out. The light passingthrough the crystal becomes polarized and when the Polaroid filter is rotated, it blocks andtransmits the two light paths in alternating fashion. The result is that the two images of PHUN canbe seen one at a time. Pretty cool stuff!

Polarization by Scattering

Polarization also occurs when light is scattered while traveling through a medium. When light strikesthe 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 thesame original frequency. These vibrating electrons produce another electromagnetic wave that isonce 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 bluenessof 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

http://www.physicsclassroom.com/Class/light/u12l2f.cfmhttp://www.physicsclassroom.com/Class/light/u12l2f.cfm

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glare in the skies. Photographers know that this partial polarization of scattered light leads tophotographs characterized by a washed-outsky. 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 isreduced. 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.

Applications of Polarization

Polarization has a wealth of other applications besides their use in glare-reducing sunglasses. Inindustry, Polaroid filters are used to perform stress analysis tests on transparent plastics. As lightpasses through a plastic, each color of visible light is polarized with its own orientation. If such a

plastic is placed between two polarizing plates, a colorful pattern is revealed. As the top plate isturned, the color pattern changes as new colors become blocked and the formerly blocked colors aretransmitted. A common Physics demonstration involves placing a plastic protractor between twoPolaroid plates and placing them on top of an overhead projector. It is known that structural stress

in plastic is signified at locations where there is a large concentration of colored bands. This locationof stress is usually the location where structural failure will most likely occur. Perhaps you wish that

a more careful stress analysis were performed on the plastic case of the CD that you recentlypurchased.

Polarization is also used in the entertainment industry to produce and show 3-D movies. Three-dimensional movies are actually two movies being shown at the same time through two projectors.The two movies are filmed from two slightly different camera locations. Each individual movie isthen projected from different sides of the audience onto a metal screen. The movies are projected

through a polarizing filter. The polarizing filter used for the projector on the left may have itspolarization axis aligned horizontally while the polarizing filter used for the projector on the rightwould have its polarization axis aligned vertically. Consequently, there are two slightly differentmovies being projected onto a screen. Each movie is cast by light that is polarized with an

orientation perpendicular to the other movie. The audience then wears glasses that have twoPolaroid filters. Each filter has a different polarization axis - one is horizontal and the other isvertical. The result of this arrangement of projectors and filters is that the left eye sees the moviethat is projected from the right projector while the right eye sees the movie that is projected from

the left projector. This gives the viewer a perception of depth.

Our model of the polarization of light provides some substantial support for the wavelike nature oflight. It would be extremely difficult to explain polarization phenomenon using a particle view of

light. Polarization would only occur with a transverse wave. For this reason, polarization is one morereason why scientists believe that light exhibits wavelike behavior.

Watch It!The pattern of a hot air balloon was sketched onto a glass plate. Cellophane tape was then added to

the pattern such that each "sector" of the balloon consisted of tape alligned in a distinctly differentdirection than adjacent "sectors". A hobby knife was used to carefully remove overlap of tape from

one sector into adjoining sectors. The cellophane tape is able to rotate the axis of polarization of thewavelengths (i.e., color) of polarized light different amounts.

In the demonstration, a polaroid filter is placed upon the glass panel of a classroom style overheadprojector. Light passing through the filter becomes polarized. Different sectors of the taped glasswill rotate the axes of polarization of the different wavelengths of light different amounts. A second

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filter is then placed over the taped glass. This second filter permits passage of wavelengths (i.e.colors) of light whose axis of polarization line up with the transmitting axis of the filter; other

wavelengths are blocked. Thus, different sectors appear different colors when viewed through bothfilters.

Check Your Understanding

1. Suppose that light passes through two Polaroid filters whose polarization axes are parallel to eachother. What would be the result?

2. Light becomes partially polarized as it reflects off nonmetallic surfaces such as glass, water, or aroad surface. The polarized light consists of waves vibrate in a plane that is ____________

(parallel, perpendicular) to the reflecting surface.

3. Consider the three pairs of sunglasses below. Identify the pair of glasses is capable of eliminating

the glare resulting from sunlight reflecting off the calm waters of a lake? _________ Explain. (Thepolarization axes are shown by the straight lines.)

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