Physics 212 Lecture 26, Slide 1

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Physics 212 Lecture 26
Lenses
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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

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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
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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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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
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Recipe for finding image
object
f
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image is real inverted smaller
S gt 2f
object
f
image
f
S
S
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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
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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
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image is virtual upright smaller
object
f
image
flt0
S
Slt0
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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
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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
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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
or the mirror !
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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

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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
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CD
The lens works due to the difference in indices
of refraction, which the difference would now be
smaller, and light bent less.
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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.
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CD
A mirror's focal length does not depend on the
index of refraction of the mirror, as only
reflection is occurring, not transmission.
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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 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

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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 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
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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 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
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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 ot obtain an image
magnified by a factor of 5 that is NOT inverted?
EQUATIONS
PICTURES
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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 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
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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
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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