Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley PowerPoint ® Lectures for University Physics, Twelfth Edition – Hugh D. Young and Roger A. Freedman Lectures by James Pazun Chapter 35 Interference

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Goals for Chapter 35 To consider interference and coherent sources To study two-source interference of light To determine intensity in interference patterns To consider interference in thin films To study the Michelson interferometer

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Introduction Rainbows in the sky: been there, seen that. A thin-film soap bubble: why should that create a rainbow effect? It seems we need to read Chapter 35 because the man on page 1207 has a large soap bubble and this thin film is dispersing white light and revealing a r.o.y.g.b.i.v. spectrum of color.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Wave fronts from a disturbance Think back to our first slide on wave motion when the father threw an object into the pool and the boy watched the ripples proceed outward from the disturbance. We can begin our discussion of interference from just such a scenario, a coherent source and the waves from it that can add (constructively or destructively). Consider Figure 35.1.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A “snapshot” The “snapshot” of sinusoidal waves spreading out from two coherent sources. Consider Figure 35.2.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Two-source interference of light Figure 35.4 shows two waves interfering constructively and destructively. Young did a similar experiment with light. See below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley As the waves interfere, they produce fringes Consider Figure 35.6 below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Interference from two slits or two radio stations Follow Example 35.1, illustrated by Figure 35.7 (lower left). Follow Example 35.2, illustrated by Figure 35.8 (lower right).

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Intensity distribution Figure 35.10, below, displays the intensity distribution from two identical slits interfering. Follow Example 35.3.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Thin films will interfere The reflections of the two surfaces in close proximity will interfere as they move from the film. Figure at right displays an explanation and a photograph of thin- film interference.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley An air wedge between two glass plates Just like the thin film, two waves reflect back from the air wedge in close proximity, interfering as they go.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Interference between mechanical and EM waves Figure compares the interference of mechanical and EM waves.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Thick films and thin films behave differently Refer to Figure in the middle of this slide. Read Problem-Solving Strategy Follow Example 35.4, illustrated by Figure at the bottom of the slide.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Thin-film examples Consider Example Consider Example 35.6, illustrated by Figure shown below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s rings Figure illustrates the interference rings resulting from an air film under a glass item.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Using fringes to test quality control An optical flat will only display even, concentric rings if the optic is perfectly ground. Follow Example 35.7.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Michelson and Morley’s interferometer In this amazing experiment at Case Western Reserve, Michelson and Morley suspended their interferometer on a huge slab of sandstone on a pool of mercury (very stable, easily moved). As they rotated the slab, movement of the earth could have added in one direction and subtracted in another, changing interference fringes each time the device was turned a different direction. They did not change. This was an early proof of the invariance of the speed of light.