Download Unit 28 - Optical Devices

Physics 2112
Unit 28
Today’s Concept:
A) Optical Devices
- multiple lenses
- human eye
- telescope
Example 28.1: Back to the spider
Remember our spider that was 15cm to the left of a
converging lens with a focal length of 20cm? A virtual
image was formed 60cm upstream of the lens. (m=4)
What if I put a second similar lens 10cm downstream
of the first one.
f
Where is the final image of the two
lens system? What is total
magnification of the two lenses?
Unit 28 - Slide2
Example 28.2: One final spider
A converging lens with |f| = 25cm is located 12cm to
the right of a diverging lens with |f| = 20cm. A 2cm
tall spider is located 60cm to the left of the
converging lens.
Where is the final image of the two
lens system? What is total
magnification of the two lenses?
Unit 28 - Slide3
System of Lenses
Virtual Objects are Possible !!
Object Distance is Negative!
Image from first lens
Becomes object for second lens
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It’s Always the Same:
1 1 1
 
S S f
S
M 
S
You just have to keep the signs straight:
s’ is positive for a real image
f is positive when it can produce a real image
Lens sign conventions
S: positive if object is “upstream” of lens
S’ : positive if image is “downstream” of lens
f: positive if converging lens
Mirrors sign conventions
S: positive if object is “upstream” of mirror
S’: positive if image is “upstream” of mirror
f: positive if converging mirror (concave)
Unit 28 - Slide5
Human Eye
As always:
1/f = 1/S + 1/S’
S changes from object to object,
but S’ is fixe by size of eye.
What to do?
Change f by changing R
S’
Close object (small S)
Small R
Distant object (large S)
Large R
“accomodation” done by ciliary muscle
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Normal Eye
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Near-Sighted (myopia)
Can’t focus beyond the “far point”.
Fix with diverging lens that creates virtual image at far point.
Unit 28 - Slide8
Example 28.1: Near Sighted
Diana can’t focus on things that are farther
that 40cm from her eye.
What should the power of her corrective
contact lenses be?
Unit 28 - Slide9
Example 28.2: Near Sighted (II)
Diana can’t focus on things that are farther
that 40cm from her eye.
Where would her image form for very distant
objects if she didn’t have her contacts in?
Unit 28 - Slide10
Far-Sighted (hyperopia)
Eye’s lens gets rigid with age (presbyopia) Muscles can create small enough R.
Converging Lens creates virtual image at person’s near point
Unit 28 - Slide11
Example 28.3: Far-Sighted
Frank can’t focus on anything closer than
1.0m from his eye.
Assuming he wants to read books that are
15cm from his eye, what should the power
of his corrective glasses be?
Assume glasses are
2cm in front of his
eye.
2cm
Unit 28 - Slide12
Specific example: Telescope
Angular magnification = b/a
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Telescope
Key point: Looking at object far away, S

M = Q2/Q1
Want big fo
Long telescopes!
 hi/fe / hi/fo
= fo/fe
Unit 28 - Slide14
Example 28.4: Yerkes
The Yerkes telescope in southern
Wisconsin is the world’s largest refracting
telescope.
What is it’s magnification
if fo = 19.8m and
fe = 3.9cm?
Unit 28 - Slide15
Real Modern Astronomical Telescope
Astronomical telescopes
don’t really care about
magnification so much.
Want large mirrors to
collect light from faint
objects.
Large aperture to
“resolve” close objects
(more next week on
resolution.)
The Large Synoptic Survey Telescope (LSST)
8.4m mirror (27.5ft)
Unit 28 - Slide16
How to Make a Big Telescope Mirror
Melt it & Spin it.
52,000 lbs of borosilicate glass when filled
Unit 28 - Slide17