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Announcements 11/12/12

Announcements 11/12/12. Prayer Labs 8 & 9 due Saturday Progress Reports – replies sent. In the Bleachers. Review. From warmup. Extra time on? How do beam splitters work? Other comments? Is there a demo on Newton's rings?

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Announcements 11/12/12

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  1. Announcements 11/12/12 • Prayer • Labs 8 & 9 due Saturday • Progress Reports – replies sent In the Bleachers Review

  2. From warmup Extra time on? How do beam splitters work? Other comments? Is there a demo on Newton's rings? I worked on a research group at my previous university. The leading professor in our group is/was a researcher at VIRGO and and LIGO. For LIGO (in the US), he said they could detect vibrations in the earth's crust that varied due to the rush hour(s) in nearby large cities. I thought that was pretty cool.

  3. Fourier Transforms • How can our two-slit analysis possibly have anything to do with Fourier transforms? (this is the y-coordinate on the slits, not the y-coordinate on the screen) compare to:

  4. Adding up phases • For an equally-spaced pattern of slits, how do the DPLs compare? • Each f is a multiple of f2! (Could have an overall reference phase for f1…not too important.) In short, we need to add up a bunch of vectors that have the same magnitude (1), but angles (phases) that go like 0, 20, 40, 60, etc. … For a different position on the screen (measured by y or q, we need to add up a different set of phases… perhaps like 0, 21, 42, 63, etc. slits screen

  5. Adding up phases, cont. • Quick writing: graphically add these three vectors: 10 + 120 + 140 • What about 10 + 190 + 1180 … slits screen

  6. Three Slit Problem: Scanning Theta Credit: this animation and the next one are from Dr. Durfee Note: for some reason he picked the overall reference phase to be about 20

  7. Clicker question: • How many “sub” peaks are there between the “main” peaks in a 5-slit interference pattern? • 1 • 2 • 3 • 4 • 5

  8. Five Slit Problem: Scanning Theta Note: for some reason he picked the overall reference phase to be about 20-30

  9. Clicker question: • When a light wave travels from a low index to a high index material at normal incidence (perpendicular to surface), what is the phase shift of the reflected wave? • 0 • 45 • 90 • 180 • depends on whether it is s- or p-polarization

  10. From warmup In Fig. 37.9 (8th edition) the textbook compares the reflection of light from a surface to the reflection of a wave on a rope. How does the analogy work? I don't have the 8th edition Light reflecting off a higher n substance is like a wave reflecting off a rigid support, and reflecting off a lower n substance is like reflecting off a free support. The n value correlates to how easily the light can move through the substance, which is similar to a rope being free vs rigid. Colton: CAUTION! this analogy only works for close to normal incidence, i.e. 0 deg. (this fact not mentioned in book).

  11. Remember these? • “Fresnel Equations” If near perpendicular (1-D problem) Same as strings The Truth (overlooked by textbook): you don’t always get a phase shift, even if going fast to slow. (Brewster marks boundary) For arbitrary angle More Truth: sometimes phase shifts not just 180: can have complex n, complex q, etc. You don’t need this much truth! 

  12. t r Air to glass (n=1 to n=1.5) p-polarization field amplitudes vs q Brewster 180 phase shift (close to perpendicular) 0 phase shift (close to glancing)

  13. Rays drawn at an angle to make viewing easier. They’re really perpendicular to surface. Back to “near normal incidence” • From low to high index: 180 phase shift • From high to low index: no phase shift • Quick writing: What does the thickness of this slab need to be to get constructive interference between the two rays? If rays at an angle… determine if above/ below Brewster angle (if p-polarization). air thin glass thickness t air

  14. From warmup The concept of "optical path length" (OPL) is used in many places to analyze optical problems. (Unfortunately your textbook doesn't use it.) See for example Wikipedia, http://en.wikipedia.org/wiki/Optical_path_length. In materials with a constant index of refraction n, the OPL is just the path length times n. In the context of today's reading, why might that be a helpful concept? I think it would help when we analyze reflections off of thin films because the destructive interference depends on the index of refraction of the film times the thickness times two. That would be the optical path length.

  15. Optical path length • OPL = Path Length  n  since wavelength inside the material is reduced by a factor of n, the distance “looks” bigger than it actually is • Constructive interference: OPL ( any phase shifts) = ml • Destructive interference: OPL ( any phase shifts) = (m+1/2) l

  16. New situation • What does the thickness of the COATING need to be to get constructive interference between the two rays? Rays drawn at an angle to make viewing easier. They’re really perpendicular to surface. air thin coating, n = 1.3 thickness t thick glass, n = 1.5

  17. Pretty pictures • What’s going on here? http://twilit.wordpress.com/2008/03/15/bubbles-and-interference/ http://superphysics.netfirms.com/pp_optics.html

  18. Demo • Demo: Soap film

  19. Interferometer • From lab 9: changing optical path length, yields ngas Interference! How does this disprove the ether?

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