Announcements 11/5/12
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Prayer
Term project progress reports due Saturday night
Slinkies: Ryan & Clement
Frank &
Ernest
From warmup
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Extra time on?
a.(nothing in particular)
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Other comments?
a.(nothing in particular)
Class-designed problem
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p = _____
Lens 1: f = _____
Distance to Lens 2 = _____
Lens 2: f = _____
5 mins for class to solve
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Ray diagram? (for first lens)
Final q = ? Real or virtual?
Mtot = ? Upright or inverted?
Quick writing
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Draw the ray diagram for this situation:
Converging lens, f = 20 cm, p = 80 cm
Quick writing
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Draw the ray diagram for this other situation:
Converging lens, f = 20 cm, p = 15 cm
Quick writing
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Draw the ray diagram for this third situation:
Diverging lens, f = -20 cm, p = 15 cm
Aberrations
Spherical Aberration
Credit: the next several slides were produced by Dr. Durfee
Reducing Spherical Aberration with
Aperture
Spherical Aberration
Reducing Spherical Aberration by
Reversing Lens
Coma
0 Degree Tilt
10 Degree Tilt
20 Degree Tilt
30 Degree Tilt
40 Degree Tilt
40 Degree Tilt + Aperture
Clicker question:
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Chromatic Aberration
Red light coming from O produces an image at I.
Where will blue light coming from O form an
image?
Lens-makers’ eqn:
a. same place
 1
1
1 
b. closer to the lens
  n  1  

f
 R1 R2 
c. farther from lens
How does a camera work?
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Simplest picture
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What if you leave out the lens?
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What’s a pinhole camera?
From warmup
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If the camera is initially focused on a distant object, to
focus on a close object , must the lens be moved
away from the CCD or moved toward the CCD?
a. Away from the CCD, because since the focal length
of the lens is constant, if p gets smaller, q must get
bigger.
F-number
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f# = The focal length divided by the
diameter of the lens
Clicker question:
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How does the f-number of a
lens relate to the half-angle
of the “light cone” formed by
parallel light entering the
lens?
a. q  2cos1 
1


f-number

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b. q  sin 1  f-number 
1



 f-number 
c. q  sin 1 
d. q  tan 1  f-number 
1 
1

e. q  tan 

2

f-number


q
From warmup
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Suppose you are comparing two cameras which
both use the same size sensor and the same zoom
level. One of them gives the aperture specification
as "f/2.8" whereas the other one is "f/2.2". Which
one allows more light to reach the sensor? (Or do
they allow the same?)
a. f/2.2, because it increases the area of aperture
b. That depends on how long the shutter is open
for each one. The one with f/2.2 will let more
light through in a given time, but if the shutter
speeds are timed differently, then in a single
picture they may let the same amount of light
through, or either one may have more.
Cameras: f-stops
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Which has the higher f-number?
What’s the advantage of one over the other?
Clicker question:
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If you have trouble seeing things far away
but can see close objects well, you are “nearsighted”. What kind of lenses do you need in
your glasses?
a. Converging lenses
b. Diverging lenses
Near point vs. Far point
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Far point: how far away can you focus on things
a. Regular person: fp = infinity
b. Nearsighted person: fp = much smaller
– For me, my fp  1 m (right eye), 0.5 m (left eye)
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Near point: how close can you see things comfortably
a. Regular person: np  25 cm
b. Farsighted person: np = much larger
How to fix things with glasses?
Nearsighted: object at  cm  image at –fp (diverging lens)
Farsighted: object at 25 cm  image at –np (converging lens)