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THE WAVE NATURE OF LIGHT

fromNEWTON

toHUYGHENS

By: Joy Augustine & John Newsome

Introduction

• This chapter extends the knowledge gained by the students in Grade 11 physics about the nature of mechanical waves.

• Interference is the property of light that clearly demonstrates the wave nature of light.

• This property of light is readily discovered by diffraction.

Background

The chapter begins with a brief description of the two models of light; particle and wave models and moves on to the experiments and observations that established the wave nature of light. Huygens developed a model that is still helpful in analyzing wave properties. Finally, Thomas Young demonstrated the diffraction of light, establishing its wave nature.

Light: Particle or Wave?Think-Pair Share Inquiry Activity

Goal: Identify the properties of light that support either the wave model or the particle (corpuscular) model.

What to Do:• Click on the links below to watch the videos on the two

models of light:Particle ModelWave Model• List of the properties of light mentioned in the two videos. For

each property, write a brief explanation of why or why not the particular property can support the wave model and/or the particle (corpuscular) model.

Particle Theory Vs. Wave Theory

• Only four properties of light; rectilinear propagation, reflection, refraction, and dispersion are explained satisfactorily by Newton’s particle theory.

• All properties of light; rectilinear propagation, reflection, refraction, partial reflection-partial refraction, diffraction are explained by Huygens’ wave theory.

INTERLUDE: INTERFERENCE OF LIGHT

WAVE THEORY of LIGHT: Diffraction & Interference

Something that can be naturally explained in a wave theory is diffraction & interference. You can see in the pictures how water waves passing through a pair of slits are “re-emitted” in the way

shown by Huyghens. The key feature is the constructive interference between the 2 resulting wavefronts in certain directions, & the

destructive interference in other directions.

LEFT: emission of waterwaves from 2 pointsources- the lower waveshave shorter wavelength.

RIGHT: Diffraction ofwavefront through a singleslit (top), and through apair of slits (bottom).

Overall Expectations

• E1. analyse technologies that use the wave nature of light, and assess their impact on society and the environment;

• E2. investigate, in qualitative and quantitative terms, the properties of waves and light, and solve related problems;

• E3. demonstrate an understanding of the properties of waves and light in relation to diffraction, refraction, interference, and polarization.

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SuperpositionConstructive Interference

+1

-1t

++1

In Phase

-1t

+2

t

-2

SuperpositionDestructive Interference

+1

-1t

++1

Out of Phase

-1t 180 degrees

+2

t

Interference Patterns

• Constructiveinterference occurs atthe center point

• The two waves travelthe same distance– Therefore, they arrive

in phase

Interference Patterns, 2

• The upper wave hasto travel farther thanthe lower wave

• The upper wavetravels onewavelength farther– Therefore, the waves

arrive in phase

• A bright fringe occurs

Interference Patterns, 3

• The upper wave travelsone-half of awavelength farther thanthe lower wave

• The trough of thebottom wave overlapsthe crest of the upperwave (180° phase shift)

• This is destructiveinterference– A dark fringe occurs

Lesson 1: Properties of WavesInvestigation

Students do the investigation on page 381 of the textbook and answer the questions.

NoteClass time can be saved by setting up the ripple tank ahead of time. The room should be darkened so that students will be able to see the interference pattern distinctly.

Evaluation

A rubric is used to evaluate students’ answers to the investigation questions.

Lesson 2: Young’s Double Slit Experiment

In this lesson, The success of Young’s double-slit experiment is explained and then applied to determine the wavelength of light. Demonstrate how the distance between the slits and the screen distance affect the interference pattern using the following applet:http://www.ub.edu/javaoptics/applets/YoungEn.html

Then students are given a handout to do an inquiry activity to derive the equation for the wavelength of light using the distance between the fringes, width of the slits, and the screen distance. The conclusion of the inquiry activity is taken up and discussed as a class.

Lesson 3: Young’s Double Slit Lab

Students are divided into groups of 2 or 3 to this lab

Hypothesis

The wavelength of a certain source can be predicted using the mathematical relationship for a two-point interference pattern.

Students are given a lab handout and the lab report is evaluated using a rubric.

Safety

• Ensure that students follow all safety precautions outlined for the ripple tank investigation as they will be working with electrical equipment near water.

• Students will be working with a laser in the Young’s double-slit lab; Encourage safe procedures at all times.

Misconception

Misconception

During the ripple tank investigation, students might think that the dark shadows on their screens are produced by the crests of the water waves and that the lighter areas are caused by the troughs.

Resolution

Explain that the crests act as convex lenses and converge the light, creating a brighter line, and the troughs act as concave lenses, diverging the light to create a darker line.

Practical and Social Implication

The property of interference of light has been used in several modern technologies.

• CDs and DVDs• Anti-reflection coating on eye glasses• Interferometer

References

1. http://www.edu.gov.on.ca/eng/curriculum/secondary/2009science11_12.pdf

2. http://www.youtube.com/watch?v=MQRub3_wqs0&feature=related

3. http://www.youtube.com/watch?v=qkkem7LQc5g4. http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=552

5. http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=5926. http://www.ub.edu/javaoptics/applets/YoungEn.html7. http://www.explorelearning.com/index.cfm?method=cResource.dspDetail&ResourceID=10438. Dick, Greg, and David Keefe. Physics 12. Whitby, Ont.: McGraw-Hill Ryerson, 2002. Print