Announcements 3/11/11






Prayer
More Slinkies?
HW & Lab with no CID
Exam going on until tomorrow
Lab 7 starts tomorrow
Term project – progress report due Sat, Mar 19.
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)?
Brewster’s angle, review


The Equation
1 
n2 
q1  tan  
 n1 
The Explanation
“Brewster’s Angle”
p-polarization
field amplitudes vs q
1.0
t
0.8
0.6
0.4
r
0.2
20
0.2

Sunglasses
40
60
80
Thought question

If you send an unpolarized beam at a piece of glass
at Brewster’s angle, what happens?
a. The reflected beam is partially polarized
b. The reflected beam is completely polarized
c. The transmitted beam is partially polarized
d. The transmitted beam is completely polarized
e. More than one of the above
What’s an image?



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

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

Use a curved mirror
On the advantages of arrows…
Reading Quiz

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

Curved mirrors



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

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

Does a concave (converging) mirror always form a
real image?
a. Yes
b. No
Rest of time: class-designed problem(s)




p = _____
f = _____
ray diagram = ?
q=?