Announcements 11/3/10
Prayer
Exam going on until tomorrow
Email: HW survey for HW 14-24, goes until Friday night
Lab 7 due Saturday night
Term project – progress report due a week from Saturday.
Grading, out of 10 pts:
a. Did you submit a progress report on time?
b. Did your progress report show evidence that you had
made significant progress on your project, and that
you were on track to complete your project on time?
c. Was your proposal less than 650 words and in the
correct format (text in body of email, figures as
attachments if needed)?
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Brewster angle demo again, for those who couldn’t see it
last time (take video)
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What’s an image?
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When you look at it, it looks like there’s an object
present (but there isn’t, really)
Real
a. The light rays you see are all coming from the
location of the image
Virtual
a. The light rays you see all just seem to be
coming from the location of the image
Reading quiz
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Is the image you see behind a mirror real or
virtual?
a. Real
b. Virtual
How can you ever get a real image to form with a mirror?
A Real Image
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Use a curved mirror
On the advantages of arrows…
Reading Quiz
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What is the focal length of a concave mirror with
a radius R?
a. R
b. -R
c. R/2
d. -R/2
e. R2
Proved in a HW problem
Demos
Saucer real image or not. Class poll: What kind of
image is this?
a. Real
b. Virtual
 Spherical mirror with hanging ball pendulum
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Curved mirrors
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Not just any curve will work
Fig 36.8
Fig 36.10
Focus:
a. every ray coming in parallel will pass through focus
b. Every ray passing through focus will exit parallel
I will always use f, not R (f=R/2)
“paraxial rays”
Where will it “focus”?
focus
useful “third ray”:
center of mirror is flat
The equation
p
ho
hi
focus
f
q
Similar triangles:
hi q

ho p
Similar triangles:
hi
f

hi  ho p
The equation:
1 1 1
 
p q f
Magnification
p
ho
hi
focus
f
Similar triangles
hi q

ho p
hi
Define M: M 
ho
q
Useful way
q
to calculate: M  
Force M to be negative if image is inverted.
p
Demo
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Inverted image in light bulb
“Convex mirror”: curved the other way
The equation:
1 1 1
 
p q f
virtual image!
 but f is negative
Numbers: f = -5.5 cm, p = 16.5 cm, q = ? -4.1 cm
Negative q: means image is in back of mirror
Thought question
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Does a concave (converging) mirror always form a
real image?
a. Yes
b. No
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.
Rest of time: class-designed problems
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p = _____
f = _____
Including some with:
1) concave “converging” mirrors (f = +17 cm, +27 cm), and
2) convex “diverging” mirrors (f = -22 cm),
which we can test out!
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ray diagram = ?
q=?