topic 2.2: particle and wave models of light · s3p-2-15 describe light as an electromagnetic wave....

TOPIC 2.2: PARTICLE AND WAVE MODELSOF LIGHT

Students will be able to:S3P-2-06 Outline several historical models used to explain the nature of light.

Include: tactile, emission, particle, wave models

S3P-2-07 Summarize the early evidence for Newton’s particle model of light.Include: propagation, reflection, refraction, dispersion

S3P-2-08 Experiment to show the particle model of light predicts that the velocityof light in a refractive medium is greater than the velocity of light in anincident medium (vr > vi).

S3P-2-09 Outline the historical contribution of Galileo, Rœmer, Huygens, Fizeau,Foucault, and Michelson to the development of the measurement of thespeed of light.

S3P-2-10 Describe phenomena that are discrepant to the particle model of light.Include: diffraction, partial reflection and refraction of light

S3P-2-11 Summarize the evidence for the wave model of light.Include: propagation, reflection, refraction, partial reflection/refraction, diffraction,dispersion

S3P-2-12 Compare the velocity of light in a refractive medium predicted by thewave model with that predicted in the particle model.

S3P-2-13 Outline the geometry of a two-point-source interference pattern, usingthe wave model.

S3P-2-14 Perform Young’s experiment for double-slit diffraction of light tocalculate the wavelength of light.

Include:

S3P-2-15 Describe light as an electromagnetic wave.S3P-2-16 Discuss Einstein’s explanation of the photoelectric effect qualitatively.S3P-2-17 Evaluate the particle and wave models of light and outline the currently

accepted view.Include: the principle of complementarity

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Topic 2: The Nature of Light • SENIOR 3 PHYSICS

Topic 2.2 – 16

SPECIFIC LEARNING OUTCOME

S3P-2-06: Outline severalhistorical models used to explainthe nature of light.Include: tactile, emission, particle,wave models

GENERAL LEARNING OUTCOMECONNECTION

Students will…Recognize that scientificknowledge is based on evidence,models, and explanations, andevolves as new evidence appearsand new conceptualizationsdevelop(GLO A2)

Notes to the Teacher Early models of light were concerned withthe source of light. Did light originate in theeyes or did objects emit light? The tactiletheory was based on the ability of the eye to“touch objects.” According to Plato, lightconsisted of filaments or streamers comingfrom the eyes. When these filaments cameinto contact with an object, sight wasestablished. The emission theory was theopposite of the tactile theory and stated thatobjects sent out light beams or particles thatwould ricochet off objects and enter the eye.The emission theory was generally acceptedover the tactile theory.

The two most successful theories of lightwere the corpuscular (or particle) theory ofSir Isaac Newton and the wave theory ofChristian Huygens. Newton’s corpusculartheory stated that light consisted ofparticles that travelled in straight lines.Huygens argued that if light were made ofparticles, when light beams crossed, theparticles would collide and cancel eachother. He proposed that light was a wave.

At the end of the 19th century, James ClerkMaxwell combined electricity, magnetism,and light into one theory. He called histheory the electromagnetic theory of light.According to Maxwell, light was anelectromagnetic wave with the sameproperties as other electromagnetic waves.Maxwell’s theory, however, was unable toexplain the photoelectric effect. In 1900,Max Planck suggested that light wastransmitted and absorbed in small bundlesof energy called “quanta.” Albert Einsteinagreed with Planck’s theory and explainedthe photoelectric effect using a particlemodel of light. The quantum theorycombines the two major theories of light,suggesting that light does not alwaysbehave as a particle and light does notalways behave as a wave.

SUGGESTIONS FOR INSTRUCTION

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Topic 2.2 – 17

SENIOR 3 PHYSICS • Topic 2: The Nature of Light


Students will…Recognize that scientificknowledge is based onevidence, models, andexplanations, and evolves asnew evidence appears and newconceptualizations develop(GLO A2)

Students can research historical models oflight: tactile, emission, particle, and wavemodels.

Students can discuss the plausibility of eachmodel, the merits of each, and theirinconsistencies or basis in the evidence.

Research Report/PresentationStudents outline the major features of earlymodels of light, discussing plausibilities anddeficiencies of each model.

SUGGESTIONS FOR INSTRUCTION SUGGESTIONS FOR ASSESSMENT

SUGGESTED LEARNING RESOURCES

PSSC Physics, 6th edition (Haber-Schaim,Uri, John H. Dodge, and James A. Walter,1991)

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Topic 2.2 – 18

SPECIFIC LEARNING OUTCOMES

S3P-2-07: Summarize the earlyevidence for Newton’s particlemodel of light.Include: propagation, reflection,refraction, dispersion

S3P-2-08: Experiment to showthe particle model of lightpredicts that the velocity of lightin a refractive medium is greaterthan the velocity of light in anincident medium (vr > vi).



Notes to the Teacher In science, models are used to makepredictions about the behaviour ofphenomena in the physical world. If themodel makes accurate predictions, weaccept the model as a valid description ofthe world. If the model encounters eventsthat are discrepant, we modify the model oruse an entirely different model. Throughouthistory there have been severalconfrontations between competing models asscientific knowledge developed. A classicexample of competing models is reflected inour understanding of the nature of light. Inaddressing these learning outcomes,students should be able to summarize theevidence and discrepant events for eachmodel.

The Particle Model of Light ExplainsRectilinear Propagation of Light Newton used the analogy of a ball to explainthe rectilinear motion of light. When a ballis thrown, it describes a parabolic pathbecause of the effect of gravity. In order tofollow a straight-line path, the ball must bethrown very quickly. Newton reasoned thatparticles of light must move at very highspeeds.

DemonstrationModel: Throw a baseball slowly and thenvery fast. At slow speeds, a curvature iseasily observed, but at high speeds the balltravels in a straight line.

Light: Make a light beam pass through acloud of chalk dust to observe that lighttravels in straight lines.

Reflection of light: Newton showed that, inan elastic collision between hard spheres,the angle of incidence equals the angle ofreflection.


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Students will…Demonstrate appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)

DemonstrationModel: Videotape a billiard ball reflectingoff a side cushion and then measure theangles of incidence and reflection.

Light: Measure the angles of incidence andreflection of light from a mirror.

Refraction: Newton explained refraction bycomparing the movement of the particles oflight with that of a ball descending aninclined plane (see diagram below).According to Newton, the particles of lightwill accelerate as they pass from air towater. Newton claimed that water attractedthe particles of light, predicting that thespeed of light would be faster in water thanin air.

carbon papercarbon paper

Class DiscussionStudents state the model and correspondingobservation for light.

Students provide examples of supportingevidence for the particle model of light.



The Best From Conceptual Physics Alive!Videodiscs, Side 3, Chapter 22

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Topic 2.2 – 20


S3P-2-07: Summarize the earlyevidence for Newton’s particlemodel of light.Include: propagation, reflection,refraction, dispersion

S3P-2-08: Experiment to showthe particle model of lightpredicts that the velocity of lightin a refractive medium is greaterthan the velocity of light in anincident medium (vr > vi).



DemonstrationModel: Students can easily reproduceNewton’s experiment to show that

= a constant, derive (Snell’s Law).

Light: Perform an experiment to trace therefraction of light through water, using acheese box and pins.

Dispersion: Newton explained dispersion bysupposing that the most refracted particles(the violet particles) had a lower mass thanthe least refracted particles (red particles).

prism

RedOrangeYellowGreenBlueVioletwhite

light

sinθisinθr

DemonstrationLight: Light can be dispersed using a prism.Note any visual evidence that the amount ofbending in the dispersed light depends uponthe colour (or wavelength) of the light.

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Topic 2.2 – 21



Students will…Demonstrate appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)

Student ActivityPlace a pair of mirrors on edge with theirfaces at 90°. Place a coin close to where themirrors meet. Notice the number of coinimages you have. Start decreasing the anglebetween the mirrors. Notice what happensto the number of coin images. Now place themirrors parallel to each other, face to faceand only a few centimetres apart, with thecoin between. Notice the number of imagesof the coin.

Senior Years Science Teachers’Handbook ActivitiesRAFTS (Senior Years Science Teachers’Handbook, page 13.23): Students write arequest for subsidy on behalf of Newtonoutlining the promise of his research.

Students complete a chart to summarize thearguments in favour of Newton’scorpuscular theory (see Appendix 2.4: Chartfor Evaluating the Models of Light). Thestudents can complete the table as the otherconcepts are examined.

Peer AssessmentStudents exchange their summary chartsand evaluate arguments in favour of thecorpuscular theory of light.


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Topic 2.2 – 22


S3P-2-09: Outline the historicalcontribution of Galileo, Rœmer,Huygens, Fizeau, Foucault, andMichelson to the development ofthe measurement of the speed oflight.



Notes to the Teacher The particle model of light predicted thatthe speed of light would be faster in waterthan in air, and the wave model predictedthe opposite. Therefore, the determinationof the speed of light was seen to be a criticalexperiment in order to decide between thewave and particle models of light.

Student ActivityStudents devise an experiment to determinethe speed of the light. What challenges dowe face? What observations must we make?What equipment would be necessary?

Students draw up, individually orcollectively, a timeline to illustrate themethods used to calculate the speed of light.They can add to the timeline as the unitprogresses.

Students use software to illustrate Rœmer’smethod of determining the finite speed oflight from the period of the moons ofJupiter. See Appendix 2.10: SimulatingRœmer’s Eclipse Timings Using StarryNight Backyard for a sample procedure.

Senior Years Science Teachers’Handbook ActivitiesResearch Project: Students research thecontributions of the following scientists tothe speed of the light: Galileo, Rœmer,Huygens, Fizeau, Foucault, and Michelson.(See Appendix 2.4: Chart for Evaluating theModels of Light.)

Jigsaw Strategy (Senior Years ScienceTeachers’ Handbook, page 3.21): Divide thestudents into groups of “experts” byassigning one of the above scientists to eachgroup. After their research, re-assign thestudents into mixed groups to share theirnew knowledge and to get information aboutthe other scientists.


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SKILLS AND ATTITUDES OUTCOMES

S3P-0-1d: Describe how scientificknowledge changes as newevidence emerges and/or newideas and interpretations areadvanced.

S3P-0-4b: Work cooperativelywith a group to identify priorknowledge, initiate andexchange ideas, proposeproblems and their solutions,and carry out investigations.



Collaborative TeamworkJigsaw Strategy: To ensure the contributionof all the members of the group, each“expert” devises questions for the othermembers.

Visual Display/Research ReportFrom their research, students create aposter, including complete name, year ofbirth and death, nationality, method,observations, inferences, conclusions, andspeed of light.

Illustrative Example Complete name: Galileo GalileiYear of birth and death: 1564–1642 Nationality: ItalianMethod: Galileo and his assistant placedthemselves on distant hills approximatelyone kilometre apart. When the assistantsaw the light of Galileo’s lantern, he was tolight his lantern. Observations: Reaction time was too long. Inferences: Galileo concluded that lightmoves too quickly to calculate its speed inthis manner.

Class DiscussionStudents use information about thehistorical development of the measurementof the speed of light to show how scienceadvances technology, and how technologyadvances science.

SUGGESTIONS FOR ASSESSMENT

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Starry Night Backyard, <www.space.com>(also available from the Manitoba TextBook Bureau (Stock # 8420) at<http://www.edu.gov.mb.ca/ks4/docs/mtbb/>

Appendix 2.6: Ole Christensen Rœmer: TheFirst Determination of the Finite Nature ofthe Speed of Light

Appendix 2.7: Ole Rœmer and theDetermination of the Speed of Light

Appendix 2.9: Becoming Familiar withIonian Eclipses

Appendix 2.10: Simulating Rœmer’s EclipseTimings Using Starry Night Software

Appendix 2.11: Contributions to theDetermination of the Speed of Light


Topic 2.2 – 24


S3P-2-10: Describe phenomenathat are discrepant to the particlemodel of light.



Notes to the Teacher Newton’s particle model of light was weakin explaining partial reflection andrefraction of light and diffraction. Questionsalso arose about propagation of light. Iflight were a particle, how could light beamspass through each other without scattering?

Teacher DemonstrationPartial reflection and refraction: Use a raybox and a glass block. As you rotate theglass block, the light ray will partiallyreflect and partially refract. At the criticalangle, the light ray will totally reflect.

Newton explained this property of light byproposing a theory of “fits,” according towhich the particles of light arriving at theborder of two mediums passed by access toeasy reflection or access to easy refraction.In other words, the particles of light mustsomehow alternately reflect and refract. Askstudents, “How, in partial reflection, does aparticle know if it should reflect or refract?”Even Newton acknowledged that hisexplanation of partial reflection andrefraction was insufficient.

Diffraction is the term used to describe thespreading of light at the edges of anobstacle. Newton’s corpuscular theoryprovided no adequate explanation ofdiffraction. Newton tried to explaindiffraction through interactions andcollisions between the particles of light andthe edges of very narrow slits engraved onglass plates. (See diagram in Topic 2.2, page28, for an analog.) However, no predictionscould be made that could be tested andmeasured empirically.

Student DemonstrationStudents look at a light through a thin slitcreated by bringing together two fingers(one observes black lines parallel with thefingers, which are interference patternsresulting from diffraction).

Senior Years Science Teachers’Handbook ActivitiesRAFTS Assignment: Students write a letteron behalf of the committee of subsidies torefuse Newton’s application for subsidy byexplaining the problems with his research.


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SKILLS AND ATTITUDES OUTCOME




Group/Peer AssessmentStudents complete the chart to summarizethe arguments disputing Newton’scorpuscular model (see Appendix 2.4: Chartfor Evaluating the Models of Light).

Students prepare a report of laboratoryexperiments designed by them toinvestigate the particle model of light.



Senior Years Science Teachers’ HandbookActivities (RAFTS)

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Topic 2.2 – 26


S3P-2-11: Summarize theevidence for the wave model oflight.Include: propagation, reflection,refraction, partial reflection/refraction, diffraction, dispersion

S3P-2-12: Compare the velocityof light in a refractive mediumpredicted by the wave model withthat predicted in the particlemodel.



Notes to the Teacher Discuss the evaluation of the two modelsof light in terms of Kuhn’s characteristicsof a good theory.• Accuracy: Do observations match the

prediction of the model?• Simplicity: Is the model simple to

understand? Do some explanationsbegin to get complicated (e.g., Newton’sexplanation of diffraction)?

• Explanatory Power: How much doeseach model explain? Are there anycontradictions within the model?

Students should have a good understandingof the properties of waves from the firsttopic. Students compare the properties ofwaves and the properties of light.

PropagationHuygens thought of light rays as thedirection of travel of the wave (wave ray).

ReflectionIt can be demonstrated using water wavesthat the angle of incidence is equal to theangle of reflection for waves.

RefractionWhile passing from the air to water, a lightray deviates towards the normal. Huygens’wave theory explained this deviation byproposing that the speed of the wavedecreases in the heavier medium. Snell’sLaw can be derived from Huygens’geometry. Huygens’ model predicts that thespeed of light is less in water than in air.This explanation goes against Newton, whopredicted that the speed of light is greaterin water. Thus, determining the speed oflight becomes a “critical experiment,” whichprovides definitive support for one theoryand eliminates the other.


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Partial Reflection/RefractionWhen one varies the angle of incidence of awave passing from one medium to another,part of the wave is reflected and part isrefracted. In the particular case where thewave passes from a slow medium to a fastmedium, there is a critical angle where theentire wave is reflected. This phenomenon iscalled internal total reflection.

Students evaluate each model according toits capacity to explain the behaviour of thelight (see Appendix 2.4: Chart forEvaluating the Models of Light).



ReferencesAppendix 2.4: Chart for Evaluating theModels of Light

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Topic 2.2 – 28


S3P-2-11: Summarize theevidence for the wave model oflight.Include: propagation, reflection,refraction, partial reflection/refraction, diffraction, dispersion

S3P-2-12: Compare the velocityof light in a refractive mediumpredicted by the wave model withthat predicted in the particlemodel.



Diffraction

BarrierBarrier

When a wave passes by a barrier, it bends(diffraction). If the wave passes through twobarriers, both ends bend. If the opening isnarrow enough (compared to thewavelength), the waves will emerge ascircular waves. In this way, as waves passthrough two thin slits, a pattern ofconstructive and destructive interferenceoccurs.

A similar pattern of interference occurs withlight. When two crests or two troughs meet,light is enhanced and a bright area can beseen. When a crest meets a trough,destructive interference occurs and a darkarea is seen. The interference pattern showsup as a series of alternating light and darkbands.

Poisson’s Spot: When light is diffractedaround a spherical object (like a steel ballbearing), a bright spot appears in themiddle of the diffraction pattern. There isan interesting story behind the discovery ofPoisson’s Spot. In 1818, Augustin Fresnelpresented a paper on the theory of


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Topic 2.2 – 29






diffraction to the French Academy. Contraryto Newton’s corpuscular model, Fresnel’stheory represented light as a wave. SiméonPoisson, a member of the judging committeefor the competition, was very critical of thewave theory of light. Using Fresnel’s owntheory, Poisson deduced that light diffractedaround a circular obstacle would produce abright spot behind the object. Poissonbelieved that this was an absurd outcomethat falsified Fresnel’s theory. However,another member of the judging committee,Dominique Arago, produced the spotexperimentally and Fresnel won thecompetition.

Senior Years Science Teachers’Handbook ActivitiesStudents compare and contrast theproperties of the waves and light.

Demonstration of the MajorExplanatory Stories of ScienceRAFTS: Students write an article thatNewton would have written to a scientificreview (such as Science and Life, Géo, orDiscover) to explain why his particle modelis superior to the wave model with regard tothe behaviour of the light. Students alsowrite the reply that Huygens would haveformulated to defend his model.


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Topic 2.2 – 32


S3P-2-16: Discuss Einstein’sexplanation of the photoelectriceffect qualitatively.


Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

Notes to the Teacher Heinrich Hertz noticed that when certainmetal surfaces are exposed to an ultravioletlight, negative charges are emitted from themetal. How can we explain the emission ofnegative charges, using the particle andwave models of light? Discuss thepredictions of the wave and particle modelswith respect to the photoelectric effect.

To dislodge an electron of a metal surface, itis necessary to communicate a certainquantity of energy to the electron.

Prediction of the Wave ModelThe wave model predicts that the energydistributed along the wave will eventuallybuild up and release a package of electronsat the same time. According to the wavemodel, light of any frequency shoulddemonstrate the photoelectric effect. Lowerfrequencies would just take longer to buildup enough energy to release the electrons. Alow frequency wave with high intensityshould eventually be able to dislodge anelectron. This, however, is not the case.Only certain frequencies emit electrons. Thewave model fails to accurately predict thephotoelectric phenomenon.

Prediction of the Particle ModelEinstein proposed that light consists ofpackets of energy called “photons,” and thatthe quantity of energy of each photon isfixed and depends on its frequency. Thus,the particle model predicts that individualphotons knock out electrons and that onlyphotons with enough energy (above thethreshold frequency) can do this.


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Students will…Understand how stability,motion, forces, and energytransfers and transformationsplay a role in a wide range ofnatural and constructedcontexts(GLO D4)

The photoelectric effect can be illustratedwith a video.

Students complete a word splash to examinethe vocabulary related to the nature ofscience.

Senior Years Science Teachers’Handbook ActivitiesStudents compare and contrast the waveand particle model’s ability to explain thephotoelectric effect.

Students draw up flow charts or conceptualcards to consolidate their comprehension ofthe vocabulary related to the nature ofscience and the models seeking to explainlight.

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Science Journal EntriesStudents plot a Venn diagram thatillustrates the properties of light that arewell explained by either one of the models,or by both.


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Topic 2.2 – 34


S3P-2-17: Evaluate the particleand wave models of light andoutline the currently acceptedview.Include: the principle ofcomplementarity


Students will…Recognize both the power andlimitations of science as a wayof answering questions aboutthe world and explainingnatural phenomena(GLO A2)

Notes to the Teacher As we evaluate the predictions of theparticle and wave models of light, we findthat each model explains some phenomenaand each model has difficulty explainingother phenomena. Initially, we thought thatthe determination of the speed of light andthe explanation of diffraction were criticalexperiments that eliminated the particlemodel in favour of the wave model.However, the photoelectric effect seems todo the opposite: it favours the particle modeland cannot be explained by the wave model.This brings up the question: “Can there everbe a critical experiment that eliminates onetheory and favours another?” The answer isthat there is no such thing as a criticalexperiment because some other explanationcould always exist. It is obvious that light isnot just a particle or a wave. Light has adual nature, a property that physicists callthe wave-particle duality of light. We cannotdraw pictures or visualize this duality. Ashumans, we are restricted to thinking onlyabout particles and waves independently.

Niels Bohr, the Danish physicist, declaredin his principle of complementarity that, “tounderstand a specific experiment, one mustuse either the wave or photon theory butnot both.” To understand light, one mustunderstand the characteristics of bothparticles and waves. The two aspects oflight complement each other.


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Students will…Evaluate, from a scientificperspective, information andideas encountered duringinvestigations and in daily life(GLO C8)

Class DiscussionStudents explain Bohr’s quote, “tounderstand a specific experiment, one mustuse either the wave or photon theory butnot both.”




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Topic 2.2 – 36


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NOTES

topic 2.2: particle and wave models of light · s3p-2-15 describe light as an electromagnetic wave....

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