Friday, July 14, 2006
Announcements:
- The real-world problems for Exam 1 have been graded and can now
be picked up. Be sure to examine the posted solution to this
problem to ensure that you understand it, and let me know if you find a
discrepancy in the grading.
- The scores on this problem ranged from 8 to 25 out of 25
points, with an average of 17/25 = 68%.
- Many students did not sign the honor pledge for this problem
and
received a zero as a consequence. If you did not cheat on this
portion
of the exam, then sign the pledge so that the score you deserve can be
recorded.
- I am missing papers from 12 students, so if you turned in a
solution but did not get it back, please let me know.
- Feedback from mid-course evaluation:
- Most students feel that this course is going OK, just too much
work (average of 35 hours per week).
- The real-world problems are the least popular part of the
course. Even though I believe there are important lessons to be
learned from these problems, I will cut back and not require any more
RWPs beyond the two already assigned.
- The physics demonstrations are popular, so I will try to
include these whenever possible.
- Some students would like to see more example problems with
explanations given in class, so I will try to accomplish this within
the limited time available.
- Several students asked to have more example problems available
with full solutions. Remember that you have access to all the
problems in the book via the Instructor's Solutions Manual that can be
checked out from either the Undergraduate Library or the Math/Physics
library for 2 hours at a time.
- Student opinion of the two instructors (Michael and myself)
averages in the excellent to good range. Thanks! This is
good to know since I will be taking the lead on the lectures through
Chapter 28, and then Michael will be conducting the lectures on the
remaing three chapters.
Assignments:
- HW27b is now due Sunday at midnight
- HW28b is due Monday at midnight.
- Exam 1 corrections are due Monday
- RWP3
(Contact Lenses) is due Monday
- Web Projects are due next Friday, July 21.
Chapter 28: Physical Optics: Interference and Diffraction
28-1: Superposition and
Interference
The adding of waves (superposition)
can result in an increased amplitude (constructive
interference) or reduced amplitude (destructive interference).
Light waves interfere much like sound waves, but
since light waves have much shorter wavelengths, the effects appear
quite different.
Monochromatic light
has only one frequency, and hence a single color.
Coherent light
waves have a constant phase relationship, which is a necessary
condition for the creation of interference patterns.
28-2: Young's Two-Slit Experiment
When light passes through two narrow slits separated
by a distance d, an interference pattern will be produced with bright
fringes at angles theta: d*sin(theta) = m*lambda, m = 0, +/-1,
+/-2,... and dark fringes at d*sin(theta) = (m-0.5)*lambda, m =
0, +/-1, +/-2,...
28-3: Interference in Reflected
Waves
Light waves reflected from different surfaces can
interfere if they are observed simultaneously.
When light is reflected from a surface with a higher
index of refraction (like a solid surface), a 180-degree phase change
(half-wavelength) occurs, but no phase change results when light
reflects from a surface with a lower index of refraction (as in total
internal reflection, i.e. fiber optics).
These effects result in color variations seen in
thin films and air wedges.
CQ11 - A film of oil on water appears dark near the
edges where it is thinnest. Estimate the index of refraction of
this oil.
28-4: Diffraction - Light
changes direction when it encounters an edge.
A single slit produces a diffraction pattern with
dark fringes located at: Wsin(theta) = m*lambda, m = +/-1,
+/-2,...
28-5: Resolution - The
ability of a visual system (like an eye or camera) to distinguish
between closely spaced objects.
A circular aperture produces a diffraction pattern
where the first dark fringe is at angle theta: sin(theta) =
1.22lambda/D.
Rayleigh's
criterion: If the angular separation between two objects
is less than 1.22lambda/D, then they will appear as a single
object. This is when the first maximum and minimum of the
adjacent diffraction patterns overlap.
28-6: Diffraction Gratings
A diffraction grating produces maxima where:
d*sin(theta) = m*lambda, m = 0, +/-1, +/-2,...
Concept
Tests