Wednesday, July 12, 2006
Announcements:
- Correction on Exam 1:
After further discussion and analysis, Michael and I agree that the
correct answer to the exam question about the charged spheres that
touch and share their total charge is answer 1 (100 uC for the larger
sphere, 50 uC for the smaller sphere). If you marked this answer,
be sure to note this when you submit your test corrections so that an
additional 2 points can be added to your exam score. If you
answered option 3 (same charge of 75 uC for each sphere), you will
still receive credit since this is the answer that Michael implied to
be correct (and which I thought was correct) based on the example
presented in class using three spheres.
Assignments:
- If you have not done so already, let me know your topic ideas for
the Web Project (even if you will not actually contribute to your
group's project).
- HW26b is due tonight at midnight.
- Read Chapter 27 and submit HW27a before class tomorrow.
Chapter 26: Geometrical Optics
26-5: The Refraction of Light
When light passes through a boundary that is more or
less optically dense it will will slow down or speed up, and can change
direction due to this change in speed, a process called refraction.
The index of
refraction is a measure of the relative optical density of a
material for a given wavelength: n = c/v
Note: Since the speed of light can never be
greater than c, n is always greater than 1, and for most materials, n
< 3
Refracted light is bent towards the normal when it
enters a more optically dense medium (slows down), and away from the
normal when it speeds up.
Law of Refraction
(Snell's Law): n1sin(theta1) = n2sin(theta2)
Total internal
reflection: When the angle of refraction exceeds 90
degrees, all of the incident light is reflected back into the denser
medium. This occurs for any incident angle greater than the critical angle: sin(thetaC) =
n2/n1
Reflected light is totally polarized parallel to the
surface when the reflected and refracted rays are at right
angles. When this occurs, the incident angle is called Brewster's angle: tan(thetaB)
= n2/n1
26-6: Ray Tracing for Lenses
A lens is an object that uses refraction to bend
light and form images.
Ray diagrams can be used to analyze a lens system,
similar to the procedure used for mirrors.
26-7: The Thin-Lens Equation
The same equations used for deterimining the
location and size of the image for a mirror applies to lenses as
well: 1/do + 1/di = 1/f, m = hi/ho = -di/do
Ponderable: Do the
following mirrors and lenses produce real or virtual images?
Bathroom mirror, rear-view mirror, video projector, rear-view mirror in
a car, telescope, magnifying glass, eyeglasses.
26-8: Dispersion and the Rainbow
The index of refraction depends on the frequency of
the light passing through a given medium. Generally higher
frequency light is refracted more than lower frequency light, which
explains how rainbows are produced from raindrops that refract and
reflect sunlight to our eyes.