Physics 212 Lecture 25, Slide 1

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Some of your comments
seems pretty okay
i think that for all the problems we answer right
in lecture, we should get scooby snacks
The bit about the black tape over the lens.
Prof. Selen can you dunk your head under water
with a converging lens and tell us what happens
in number 7?
Questions like 7 and how much I dislike ECE, I
feel like everyone in the major should just vent.
Majorly.
you were right, the stuff DID get easier!!
the lens underwater business
Lense maker's law, but I'm amazed that overall I
understand almost everything in the prelecture.
Finally I unerstand a topic!!!!!!!!!!!!!!
Just a general review is good, I couldn't really
concentrate on the prelecture this time. (A
review of definitions like 'focal length' would
be nice, though).
I'm really confused. Please go over everything.
How delicious Mom's cooking is about to be on
thanksgiving!!
I actually stopped playing Guitar Hero to come do
this. I'm proud of myself.
Please talk more about ASTIGMATISM. Most of us
are nerds and wear glasses, it would be nice to
learn a "Physics-perspective" other than
"heredity" which we usually hear.
For the last several years, I've gone about not
understanding why pictures of the lenses always
show an inversion and the sort. Perhaps you can
change that. All I ask of you is to explain it in
a comprehensible way. Will you be up to the task
professor? Or will you fail just like all the
others? You have 50 minutes. Make your choice.
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Physics 212 Lecture 25 Lenses
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Refraction

• Snells Law
• n1sin(q1) n2sin(q2)

q1
n1
n2
q2
Thats all of the physics everything else is
just geometry!
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Object Location

• Light rays from sun bounce off object and go in
  all directions
• Some hits your eyes

We know objects location by where rays come from.
We will discuss eyes in lecture 27
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Converging Lens Consider the case where the
shape of the lens is such that light rays
parallel to the axis of the mirror are all
focused to a common spot a distance f behind of
the lens
f
f
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Recipe for finding image
1) Draw ray parallel to axis
refracted ray goes through focus
2) Draw ray through center
refracted ray is symmetric
object
f
image
You now know the position of the same point on
the image
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image is real inverted smaller
S gt 2f
object
f
image
f
S
S
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image is real inverted same size
S 2f
object
f
image
f
S
S
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2f gt S gt f
image is real inverted bigger
object
f
image
f
S
S
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S f
image is at infinity
object
f
f
S
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0 lt S lt f
image is virtual upright bigger
image
f
object
f
S
Slt0
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Diverging Lens Consider the case where the shape
of the lens is such that light rays parallel to
the axis of the mirror are all focused to a
common spot a distance f in front of the mirror
f
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image is virtual upright smaller
object
f
image
flt0
S
Slt0
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Executive Summary - Lenses
real inverted smaller
S gt 2f
real inverted bigger
2f gt S gt f
converging
f
virtual upright bigger
f gt S gt 0
virtual upright smaller
S gt 0
diverging
f
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Its always the same
You just have to keep the signs straight
Here are two simple rules to remember the signs
1) S is positive 2) f and S are ve on the side
where the light goes after hitting the lens,
and ve on the other side !
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image
object
Light from top of object
image
object
Light from bottom of object

• What would happen to the image if I covered up
  the top half of the lens?
• It doesnt change at all
• It completely vanishes
• The top half of the image vanishes
• Its all still there, it just gets dimmer

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air
water
qi
qi
CD
glass
glass
?
?
Case A
Case B
In Case A light in air heads toward a piece of
glass with incident angle qi In Case B, light in
water heads toward a piece of glass at the same
angle. In which case is the light bent most as
it enters the glass? A or B
sin(q2)/sin(q1) n1/n2
n1sin(q1) n2sin(q2)
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CD
light doesn't refract as much when the lens goes
underwater because the change in indices of
refraction is smaller than with a glass-air
interface.
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We can also see this from the Lensmakers
Formula
nlens
nair
This is why you can make thinner glasses out of
special materials that have n greater than nglass.
Recall glass rod in oil demo n was the same and
you saw no refraction at all
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Thanksgiving connectionwill show you before
Thursday lecture
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Calculation
A magnifying glass is used to read the fine print
on a document. The focal length of the lens is
10mm. At what distance from the lens must the
document be placed in order to obtain an image
magnified by a factor of 5 that is NOT inverted?

• Conceptual Analysis
• Lens Equation 1/s 1/s 1/f
• Magnification M -s/s
• Strategic Analysis
• Consider nature of image (real or virtual?) to
  determine relation between object position and
  focal point
• Use magnification to determine object position

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A magnifying glass is used to read the fine print
on a document. The focal length of the lens is
10mm. At what distance from the lens must the
document be placed in order to obtain an image
magnified by a factor of 5 that is NOT inverted?

• Is the image real or virtual?
• (A) REAL
  (B) VIRTUAL

A virtual image will be upright
A real image would be inverted
h
h
h
f
h
f
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A magnifying glass is used to read the fine print
on a document. The focal length of the lens is
10mm. At what distance from the lens must the
document be placed in order to obtain an image
magnified by a factor of 5 that is NOT inverted?

• How does the object distance compare to the
  focal length?
• (A) (B)
  (C)

Lens equation
h
h
Virtual Image ? s lt 0
Real object ? s gt 0
f
s
Converging lens ? f gt 0
s
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A magnifying glass is used to read the fine print
on a document. The focal length of the lens is
10mm. At what distance from the lens must the
document be placed in order to obtain an image
magnified by a factor of 5 that is NOT inverted?

• What is the magnification M in terms of s and f?
  
• (A) (B)
  (C) (D)

Magnification equation
Lens equation
h
h
f
s
s
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A magnifying glass is used to read the fine print
on a document. The focal length of the lens is
10mm. At what distance from the lens must the
document be placed in order to obtain an image
magnified by a factor of 5 that is NOT inverted?
(A) 1.7mm (B) 6mm
(C) 8mm (D) 40 mm (E) 60 mm
h
h
f