Announcements 3/14/11
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Prayer
Exam is being picked up from Testing Center even
as we speak, hopefully graded within a couple of
days
Thought question
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I look at my image in a round Christmas
ornament. Which of the following will be true?
a. The image will be upright (not inverted) and
real.
b. The image will be upright and virtual.
c. The image will be inverted and real.
d. The image will be inverted and virtual.
e. Can’t say… it will depend on where you (the
object) are relative to the focus.
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
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Demo: “Real image or not?” Class poll: What
kind of image is this?
a. Real
b. Virtual
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!!!


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=?