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Physics 212 Lecture 26, Slide 1

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Title: Physics 212 Lecture 26, Slide 1


1
Physics 212 Lecture 26
Lenses
2
Music
  • Who is the Artist?
  • B. B. King
  • Johnny Copeland
  • Elmore James
  • Luther Allison
  • Gatemouth Brown

Why? I, thinking New
Orleans 13 days, but whos counting?? In a
word, UNIQUE Many genres (Jazz. Blues. Country,..)
3
Notes from St. Louis
  • FUN
  • Broadway Oyster House
  • Tim Session (timsession.com) check it out..
    Download from media link)
  • WORK
  • Washington Tutorials (PHYS 211) Compare to
    Colorado
  • Likert Scale strongly disagree (A) -gt neutral
    (C) -gt strongly agree (E)
  • Tutorials are helpful for learning the material
    in PHYS 211
  • I enjoy tutorials (PHYS 211)
  • Working in groups in discussion section is
    helpful for learning the material
  • I enjoy working in groups in discussion section
  • The in-class clicker questions are helpful for
    learning the material
  • I enjoy in-class clicker questions

4
Your thoughts
Lets talk about water
Is there a difference between what is "virtual"
and "real" with respect to lenses and mirrors? I
just assumed they would be the same thing.
Well, it's the end of the year, so I might as
well write something profound. Physics may be Fun
However Sleep is greater Summer awaits us
All of this stuff was a bit confusing. maybe if
we could do some CALCULATION examples, to make
sure we are learning the signs of s', s, f etc
correctly ) )
Mats, I wish you were my Uncle!
Will do Fall/04 exam in next Tuesdays
lecture Final HW assignment delayed by a week
5
Refraction
  • Snells Law
  • n1sin(q1) n2sin(q2)

q1
n1
n2
q2
Thats all of the physics everything else is
just geometry!
6
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
7
Recipe for finding image
object
f
8
image is real inverted smaller
S gt 2f
object
f
image
f
S
S
9
2fgt S gt f
image is real inverted bigger
object
f
image
f
S
S
10
S f
image is at infinity
Same thing happens with a concave mirror
object
f
f
S
11
0 lt S lt f
image is virtual upright bigger
image
f
object
f
S
Slt0
12
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
13
image is virtual upright smaller
object
f
image
flt0
S
Slt0
14
Executive Summary - Lenses
real inverted bigger
2f gt S gt f
virtual upright bigger
f gt S gt 0
virtual upright smaller
S gt 0
diverging
15
Executive Summary - Mirrors
real inverted bigger
2f gt S gt f
concave
f
virtual upright bigger
f gt S gt 0
virtual upright smaller
S gt 0
convex
f
16
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
or the mirror !
17
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18
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19
CD
Object
Image
  • What would happen to the image if I covered up
    the top half of the lens?
  • It doesnt change at all
  • It completely vanishes
  • It gets dimmer

20
(No Transcript)
21
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
22
CD
The lens works due to the difference in indices
of refraction, which the difference would now be
smaller, and light bent less.
23
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.
24
CD
A mirror's focal length does not depend on the
index of refraction of the mirror, as only
reflection is occurring, not transmission.
25
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 ot 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

26
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 ot obtain an image
magnified by a factor of 5 that is NOT inverted?
  • Is the image real or virtual?
  • (A) REAL
    (B) VIRTUAL

How to get VIRTUAL image?
Real image of real object
? INVERTED image
27
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 ot obtain an image
magnified by a factor of 5 that is NOT inverted?
EQUATIONS
PICTURES
Real object ? s gt 0
Converging lens ? f gt 0
Lens equation
Virtual image
Virtual image ? s lt 0
28
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 ot obtain an image
magnified by a factor of 5 that is NOT inverted?
EQUATIONS
PICTURES
29
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 ot obtain an image
magnified by a factor of 5 that is NOT inverted?
What is the object distance s?
(A) 1.7mm (B) 6mm (C) 8mm
(D) 40 mm (E) 60 mm
EQUATIONS
PICTURES
30
Follow Up
Suppose we replace the converging lens with a
diverging lens with focal length of 10mm. If
we still want to get an image magnified by a
factor of 5 that is NOT inverted, how does the
object sdiv compare to the original object
distance sconv?
  • (A) sdiv lt sconv (B) sdiv sconv
    (C) sdiv gt sconv (D) sdiv doesnt
    exist

EQUATIONS
PICTURES
s negative ? not real object
Draw the rays s will always be smaller than
s Magnification will always be less than 1
31
Follow Up
Suppose we replace the converging lens with a
diverging lens with focal length of 10mm. What
is the magnification if we place the object at s
8mm?
EQUATIONS
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