Post on 19-Dec-2015
Lecture 30
Friday, April 17Review two slits, gratings
Thin Film Interference
Two slit interference
• Bright fringes occur at angles
• On the observing screen,
sin mm d
tanm my L
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.
0n n
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
The Intensity Pattern Due to a Diffraction Grating
Slide 17-14
The Fringes Become Very Narrow as the Number of Slits is Increased
Slide 17-15
All waves spread out after passing through a small enough gap in a barrier. This phenomenon is
known as
Ant
irefle
ction
dou
ble-sli
t in...
Refr
actio
n
diff
ract
ion
25% 25%25%25%1. Antireflection
2. double-slit interference
3. Refraction
4. diffraction
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 y5 – y1, find λ• 10: two slits given fringe spacing, λ• 12: Δr
Thin-Film Interference
Slide 17-17
Phase Changes Due to Reflection
Slide 17-18
Workbook Problem 10-11
• Wave reflected at various interfaces
A film of oil (noil = 1.2) floats on top of an unknown fluid “X” (with unknown index of refraction nx). 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?
nx >
1.2
nx <
1.2
nx =
1.2
Ther
e is i
nsuf..
.
25% 25%25%25%
1. nx > 1.2
2. nx < 1.2
3. nx = 1.2
4. There is insufficient information to tell.
Assignment for Monday
• Enjoy your weekend