# Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley PowerPoint ® Lectures for University Physics, Twelfth Edition – Hugh D. Young.

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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 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).

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 sources S 1 and S 2 oscillating in phase emit sinusoidal waves. Point P is 7.3 wavelengths from source S 1 and 4.3 wavelengths from source S 2. As a result, at point P there is Q35.1 A. constructive interference. B. destructive interference. C. neither constructive nor destructive interference. D. not enough information given to decide.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Two sources S 1 and S 2 oscillating in phase emit sinusoidal waves. Point P is 7.3 wavelengths from source S 1 and 4.3 wavelengths from source S 2. As a result, at point P there is A35.1 A. constructive interference. B. destructive interference. C. neither constructive nor destructive interference. D. not enough information given to decide.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Two sources S 1 and S 2 oscillating in phase emit sinusoidal waves. Point P is 7.3 wavelengths from source S 1 and 4.6 wavelengths from source S 2. As a result, at point P there is Q35.2 A. constructive interference. B. destructive interference. C. neither constructive nor destructive interference. D. not enough information given to decide.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Two sources S 1 and S 2 oscillating in phase emit sinusoidal waves. Point P is 7.3 wavelengths from source S 1 and 4.6 wavelengths from source S 2. As a result, at point P there is A35.2 A. constructive interference. B. destructive interference. C. neither constructive nor destructive interference. D. not enough information given to decide.

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 Interference from two radio stations Radio station operating at 1500 kHz has two antennas spaced 400m apart. In which directions is the intensity greatest in the resulting radiation pattern far away (>> 400m) from the antennas?

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 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 In Young’s experiment, coherent light passing through two slits (S 1 and S 2 ) produces a pattern of dark and bright areas on a distant screen. If the wavelength of the light is increased, how does the pattern change? Q35.3 A. The bright areas move closer together. B. The bright areas move farther apart. C. The spacing between bright areas remains the same, but the color changes. D. any of the above, depending on circumstances E. none of the above

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley In Young’s experiment, coherent light passing through two slits (S 1 and S 2 ) produces a pattern of dark and bright areas on a distant screen. If the wavelength of the light is increased, how does the pattern change? A35.3 A. The bright areas move closer together. B. The bright areas move farther apart. C. The spacing between bright areas remains the same, but the color changes. D. any of the above, depending on circumstances E. none of the above

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley In Young’s experiment, coherent light passing through two slits (S 1 and S 2 ) produces a pattern of dark and bright areas on a distant screen. What is the difference between the distance from S 1 to the m = +3 bright area and the distance from S 2 to the m = +3 bright area? Q35.4 A. three wavelengths B. three half-wavelengths C. three quarter-wavelengths D. not enough information given to decide

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley In Young’s experiment, coherent light passing through two slits (S 1 and S 2 ) produces a pattern of dark and bright areas on a distant screen. What is the difference between the distance from S 1 to the m = +3 bright area and the distance from S 2 to the m = +3 bright area? A35.4 A. three wavelengths B. three half-wavelengths C. three quarter-wavelengths D. not enough information given to decide

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 35.11 at right displays an explanation and a photograph of thin- film interference.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Interference between mechanical and EM waves Figure 35.13 compares the interference of mechanical and EM waves.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Soap bubble You want to make a soap bubble that will primarily reflect red light (700 nm wavelength in vacuum). How thick should the bubble be? Index of refraction of soapy water n = 1.33. How could you reflect blue light? (no numbers, just explain)

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 A. /2.B. 3 /4. C.  D. either A. or C. E. any of A., B., or C. Q35.6 An air wedge separates two glass plates as shown. Light of wavelength strikes the upper plate at normal incidence. At a point where the air wedge has thickness t, you will see a bright fringe if t equals

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A35.6 A. /2.B. 3 /4. C.  D. either A. or C. E. any of A., B., or C. An air wedge separates two glass plates as shown. Light of wavelength strikes the upper plate at normal incidence. At a point where the air wedge has thickness t, you will see a bright fringe if t equals

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Thick films and thin films behave differently Refer to Figure 35.14 in the middle of this slide. Read Problem-Solving Strategy 35.1. Follow Example 35.4, illustrated by Figure 35.15 at the bottom of the slide.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Thin-film examples Consider Example 35.5. Consider Example 35.6, illustrated by Figure 35.16 shown below.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Newton’s rings Figure 35.17 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.