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Announcements 11/11/11

Announcements 11/11/11. Prayer Lab 8 going on, due on Sat Nov 19 Lab 9 starts tomorrow, due on Sat Nov 19 Lab 10 starts tomorrow, due on Tues Nov 22 Slinky (down to you, Thomas…). Two facts you should know:. (They follow quickly from e ix = cosx + i sinx). Reading Quiz.

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Announcements 11/11/11

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  1. Announcements 11/11/11 • Prayer • Lab 8 going on, due on Sat Nov 19 • Lab 9 starts tomorrow, due on Sat Nov 19 • Lab 10 starts tomorrow, due on Tues Nov 22 • Slinky (down to you, Thomas…) Two facts you should know: (They follow quickly from eix = cosx + i sinx)

  2. Reading Quiz • According to the book, a double-slit diffraction pattern (viewed on a screen far from the slits) is: • A series of equal amplitude peaks, equally spaced • A series of equal amplitude peaks, alternating between two spacing distances • A series of alternating amplitude peaks, equally spaced • A series of alternating amplitude peaks, alternating between two spacing distances

  3. Two slit interference From Wikipedia http://en.wikipedia.org/wiki/Double-slit_experiment

  4. intensity screen here Double slit experiment aka “Young’s Double Slit” • Exactly the same as the two speaker demo • Goal: what’s the shape of that curve? How can we predict where the maxima & minima will be? min max min max

  5. Wait, which is correct? Experimental challenge • How do you get two points sources of light that are oscillating in phase with each other? • How did we do it with sound? • Options for light? (I can only think of two) • What he did: http://en.wikipedia.org/wiki/ Young's_Double_Slit_Interferometer

  6. How to solve the problem • Complex numbers!! • The light from each slit travels a different distance • This creates a phase shift • Incorporate the phase shift into eif • First: what’s the phase shift for two waves oscillating in phase with a known DPL? f = ( DPL / l )  360 f = 2pDPL / l

  7. What’s DPL? f = 2pDPL / l • Clicker Vote: What should we measure the path length relative to? • The top slit • The bottom slit • Halfway between the two slits Approximation #1: d is small enough that the two rays are parallel. Requires d << L.

  8. The Easy Part • Maxima/minima Maxima: DPL = ? Minima: DPL = ?

  9. The Full Answer f = 2pDPL / l • Etot = Etop slit + Ebottom slit = [use whiteboard] • I ~ |E|2 I = [use whiteboard] • Plot of I(y) for I0=1, l=500 nm, L=1 m, d=1mm • How did I turn q into y? • Approximation #2: sometimes (not here!) q is small enough that q  y/L. Requires y << L.

  10. Plots (approx. 1 and 2) (approx. 1 only)

  11. intensity screen here min max min max

  12. max min max min max min max screen here

  13. Maxima/minima, revisited • Max: cosx = 1  … • Min: cosx = 0  …

  14. What you need to know Not given on exam Given on exam • HW 32-4: Solve the three slit problem • Exam problem likely (possibly extra credit) • Disclaimer: This only works for very narrow slits  Lecture after next: wide slits Demo: Double slit experiment!

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