Announcements 11/5/10
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Prayer
Exam being graded, hopefully done by Monday
Survey about Waves HW problems. Only 7
responses so far. (Thanks!) If you are willing to
take the survey, please get your responses in by
tonight.
Found PpP book
Quick writing
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Ralph is confused about the difference between
real and virtual images. What can you say to help
him understand?
Reading Quiz
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A converging lens should be:
a. convex
b. concave
Convex/converging Lens
at each interface,
Snell’s law is
satisfied
f
parallel rays
converge to the
focal point
if not parallel, they
converge (image formed)
at a different point
Where will image be formed?
focus
focus
Ray diagram just like one for concave mirror, except
rays continue to right instead of reflecting back!
The equation:
1 1 1
 
p q f
positive q = right
hand side of lens
Magnification same:
q
M 
p
Concave/diverging Lens
f
parallel rays seem to be
coming from the focal point
Diverging lenses
virtual image!
The equation:
1 1 1
 
p q f
(negative f)
Negative q: means image on same side
of lens as object (virtual)
Demos
Blackboard optics
 Real image or not. Class poll: What kind of
image is this?
a. Real
b. Virtual
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Sign conventions: summary
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Assuming that the light rays are coming from the left:
quantity
p
q
f
M
positive
+
if on the left of the
lens/mirror
if on the right of the lens
(left side of mirror),
means a real image
if converging: convex
lens/concave mirror
if image is right-side up
negative

if on the right of the
lens/mirror (can only happen
in a compound problem)
if on the left of the lens
(right side of mirror),
means a virtual image
if diverging: concave
lens/convex mirror
if image is upside-down
Reading Quiz
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When you look straight down at an object under
water, it appears to be ____________ than
actuality.
a. closer to you
b. farther from you
Thick lenses and surfaces
n1 n2 n2  n1 Careful with signs!!!
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p q
R
(see table 36.2)
Lens-makers’ eqn:
 1
1
1 
  n  1  

f
 R1 R2 
Thought question
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Can a converging lens ever produce a virtual
image?
a. Yes
b. No
Thought question
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What happens if I cover up the upper half of lens?
a. Nothing
b. Upper half of image disappears
c. Lower half of image disappears
d. Intensity of image goes down by 50%
Thought question
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Can a diverging lens ever produce a real image?
a. Yes
b. No
Multiple element problems
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Image of one becomes object of next
Worked problem: Find qfinal, Mtot
f = 20 cm
f = -60 cm
60 cm
15 cm
Answers: image is 20 cm to right of -60cm lens; M = -0.67
Rest of time: class-designed problems
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p = _____
f = _____
Including some with these converging lenses,
which we have:
f = +8.05 cm
f = +9.34 cm
f = +17.0 cm
f = +20.8 cm
f = +45.0 cm
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ray diagram = ?
q=?