1. Lecture 30 Friday, April 17 Review two slits, gratings Thin Film Interference

2. Two slit interference Bright fringes occur at angles On the observing screen,

3. Workbook Problems Graphical representation WB 17: 1-9 1 and 2: 3: Shadow when d>>λ 4: Two slit diffraction to compare with 3 5: Graphical picture of two slit diffraction paths.

4. Grating Problems **6: Graphical path differences for four slits 7. What happens when wavelength is doubled? 8: What happens when slit width is doubled? 9. Δr = 2λ and ½λ

5. The Intensity Pattern Due to a Diffraction Grating Slide 17-14

6. The Fringes Become Very Narrow as the Number of Slits is Increased Slide 17-15

7. All waves spread out after passing through a small enough gap in a barrier. This phenomenon is known as 1.Antireflection 2.double-slit interference 3.Refraction 4.diffraction

8. Problems due Today 17: 1, 2, 4, 7, 9, 10, 12 2: speed in glass 4: wavelength in glass 7: two slit interference 9: ditto, given d and y 5 – y 1, find λ 10: two slits given fringe spacing, λ 12: Δr

9. Thin-Film Interference Slide 17-17

10. Phase Changes Due to Reflection Slide 17-18

11. Workbook Problem 10-11 Wave reflected at various interfaces

12. A film of oil (n oil = 1.2) floats on top of an unknown fluid “X” (with unknown index of refraction n x ). The thickness of the oil film is known to be very small, on the order of 10 nm. A beam of white light illuminates the oil from the top, and you observe that there is very little reflected light, much less reflection than at an interface between air and X. What can you say about the index of refraction of X? 1.n x > 1.2 2.n x < 1.2 3.n x = 1.2 4.There is insufficient information to tell.

13. Assignment for Monday Enjoy your weekend