TOPIC 2.2: PARTICLE AND WAVE MODELS OF LIGHT

1 TOPIC : PARTICLE AND WAVE MODELSOF LIGHTS tudents will be able to:S3P-2-06 Outline several historical MODELS used to explain the nature of : tactile, emission, PARTICLE , wave modelsS3P-2-07 Summarize the early evidence for Newton s PARTICLE model of : propagation, reflection, refraction , dispersionS3P-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, R mer, 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 : diffraction, partial reflection and refraction of lightS3P-2-11 Summarize the evidence for the wave model of : propagation, reflection, refraction , partial reflection/ refraction , diffraction,dispersionS3P-2-12 Compare the velocity of LIGHT in a refractive medium predicted by thewave model with that predicted in the PARTICLE Outline the geometry of a two-point-source interference pattern, usingthe wave Perform Young s experiment for double-slit diffraction of LIGHT tocalculate the wavelength of.

2 S3P-2-15 Describe LIGHT as an electromagnetic Discuss Einstein s explanation of the photoelectric effect Evaluate the PARTICLE and wave MODELS of LIGHT and outline the currentlyaccepted : the principle of complementarityxdL = TOPIC 2: The Nature of LIGHT SENIOR3 PHYSICST opic 16 SPECIFICLEARNINGOUTCOMES3P-2-06: Outline severalhistorical MODELS used to explainthe nature of : tactile, emission, PARTICLE ,wave modelsGENERALLEARNINGOUTCOMECONNECTIONS tudents 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 ?

3 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 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 argued that if LIGHT were made ofparticles, when LIGHT beams crossed, theparticles would collide and cancel eachother.

4 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 to Maxwell, LIGHT was anelectromagnetic wave with the sameproperties as other electromagnetic 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 FORINSTRUCTION TOPIC 17 SENIOR3 PHYSICS TOPIC 2: The Nature of LightGENERALLEARNINGOUTCOMECONNECTIONS tudents 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 can discuss the plausibility of eachmodel, the merits of each, and theirinconsistencies or basis in the Report/PresentationStudents outline the major features of earlymodels of LIGHT , discussing plausibilities anddeficiencies of each FORINSTRUCTIONSUGGESTIONS FORASSESSMENTSUGGESTEDLEARNINGRESOURCESP SSC Physics, 6th edition(Haber-Schaim,Uri, John H.)

5 Dodge, and James A. Walter,1991) TOPIC 2: The Nature of LIGHT SENIOR3 PHYSICST opic 18 SPECIFICLEARNINGOUTCOMESS3P-2-07: Summarize the earlyevidence for Newton s particlemodel of : propagation, reflection, refraction , dispersionS3P-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).GENERALLEARNINGOUTCOMECONNECTIONS tudents that scientificknowledge is based on evidence, MODELS , and explanations, andevolves as new evidence appearsand new conceptualizationsdevelop(GLO A2)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.

6 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 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 : Throw a baseball slowly and thenvery fast.

7 At slow speeds, a curvature iseasily observed, but at high speeds the balltravels in a straight : Make a LIGHT beam pass through acloud of chalk dust to observe that lighttravels in straight of LIGHT : Newton showed that, inan elastic collision between hard spheres,the angle of incidence equals the angle ofreflection. SUGGESTIONS FORINSTRUCTION TOPIC 19 SENIOR3 PHYSICS TOPIC 2: The Nature of LightGENERALLEARNINGOUTCOMECONNECTIONS tudents 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 : Measure the angles of incidence andreflection of LIGHT from a : Newton explained refraction bycomparing the movement of the particles oflight with that of a ball descending aninclined plane (see diagram below).

8 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 papercarbon paperClass DiscussionStudents state the model and correspondingobservation for provide examples of supportingevidence for the PARTICLE model of FORINSTRUCTIONSUGGESTIONS FORASSESSMENTSUGGESTEDLEARNINGRESOURCEST he Best From Conceptual Physics Alive!Videodiscs, Side 3, Chapter 22 TOPIC 2: The Nature of LIGHT SENIOR3 PHYSICST opic 20 SPECIFICLEARNINGOUTCOMESS3P-2-07: Summarize the earlyevidence for Newton s particlemodel of : propagation, reflection, refraction , dispersionS3P-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).

9 GENERALLEARNINGOUTCOMECONNECTIONS tudents that scientificknowledge is based on evidence, MODELS , and explanations, andevolves as new evidence appearsand new conceptualizationsdevelop(GLO A2)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 : Newton explained dispersion bysupposing that the most refracted particles (the violet particles ) had a lower mass thanthe least refracted particles (red particles ).prismRedOrangeYellowGreenBlue Violetwhite lightsin isin rDemonstrationLight: LIGHT can be dispersed using a any visual evidence that the amount ofbending in the dispersed LIGHT depends uponthe colour (or wavelength) of the er i rNo refraction occurshere since i = 0 O SUGGESTIONS FORINSTRUCTION TOPIC 21 SENIOR3 PHYSICS TOPIC 2: The Nature of LightGENERALLEARNINGOUTCOMECONNECTIONS tudents appropriatescientific inquiry skills whenseeking answers to questions(GLO C2)Student ActivityPlace a pair of mirrors on edge with theirfaces at 90.

10 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 Years Science Teachers HandbookActivitiesRAFTS (Senior Years Science Teachers Handbook, page ): Students write arequest for subsidy on behalf of Newtonoutlining the promise of his complete a chart to summarize thearguments in favour of Newton scorpuscular theory (see Appendix : Chartfor Evaluating the MODELS of LIGHT ). Thestudents can complete the table as the otherconcepts are AssessmentStudents exchange their summary chartsand evaluate arguments in favour of thecorpuscular theory of FORINSTRUCTIONSUGGESTIONS FORASSESSMENT SUGGESTEDLEARNINGRESOURCESThe Best From Conceptual Physics Alive!