x THE WAVE NATURE OF LIGHT from NEWTON to HUYGHENS 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 Model Wave 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 water waves from 2 point sources- the lower waves have shorter wavelength. RIGHT: Diffraction of wavefront through a single slit (top), and through a pair 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.

-2 Superposition Constructive Interference +1 t + +1 In Phase t +2 t

-2 Superposition Destructive Interference +1 t + +1 Out of Phase t 180 degrees +2 t

Interference Patterns Constructive interference occurs at the center point The two waves travel the same distance – Therefore, they arrive in phase

Interference Patterns, 2 The upper wave has to travel farther than the lower wave The upper wave travels one wavelength farther – Therefore, the waves arrive in phase A bright fringe occurs

Interference Patterns, 3 The upper wave travels one-half of a wavelength farther than the lower wave The trough of the bottom wave overlaps the crest of the upper wave (180° phase shift) This is destructive interference – A dark fringe occurs

Lesson 1: Properties of Waves Investigation Students do the investigation on page 381 of the textbook and answer the questions. Note Class 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: 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 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 Dick, Greg, and David Keefe. Physics 12. Whitby, Ont.: McGraw-Hill Ryerson, Print