263 265 268 Physics 1161 Lecture 17 Refraction amp Lenses Physics 1161 Lecture 17 Slide 2 Indices of Refraction Checkpoint Refraction n 1 n 2 When light travels from one medium to another the speed changes vcn but the frequency is constant So the light be ID: 384168
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Slide1
Textbook sections 26-3 – 26-5, 26-8
Physics 1161: Lecture 17
Refraction & Lenses
Slide2
Physics 1161: Lecture 17, Slide 2Indices of RefractionSlide3
CheckpointRefraction
n
1
n
2
When light travels from one medium to another the speed changes v=c/n, but the frequency is constant. So the light bends:
q
1
q
2
1) n
1
> n
2
2) n
1
= n
2
3) n
1
< n
2
Compare n
1 to n2.Slide4
n
1
n
2
Compare n
1
to n
2
.
q
2
q
1
1) n
1
> n
2
2) n
1
= n
2
3) n
1 < n
2q1
<
q
2
sin
q
1
< sin
q
2
n
1
> n
2
Which of the following is correct?
n
1
sin(
q
1
)= n
2
sin(
q
2)
Checkpoint
Refraction Slide5
A ray of light crossing the boundary from a fast medium to a slow medium bends toward the normal.
(FST)A ray of light crossing the boundary from a slow medium to a fast medium bends away from the normal. (SFA)
FST & SFASlide6
n
1
n
2
Snell’s Law Practice
normal
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is
reflected
at an angle
q
r
= 60. The other part of the beam is refracted. What is
q
2
?
1
r
Usually, there is both
reflection
and
refraction
!
ExampleSlide7
n
1
n
2
Snell’s Law Practice
normal
A ray of light traveling through the air (n=1) is incident on water (n=1.33). Part of the beam is reflected at an angle
q
r
= 60. The other part of the beam is refracted. What is
q
2
?
sin(60) = 1.33 sin(
q
2
)
q
2
= 40.6 degrees
q
1
=
q
r
=
60
1
r
Usually, there is both
reflection
and
refraction
!
ExampleSlide8
Refraction AppletsApplet by Molecular Expressions -- Florida State UniversityApplet by Fu-Kwung Hwang, National Taiwan Normal UniversitySlide9
Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?
1
air
air
2
Medium 1
Medium 2
Both the sameSlide10
Parallel light rays cross interfaces from air into two different media, 1 and 2, as shown in the figures below. In which of the media is the light traveling faster?
1
air
air
2
Medium 1
Medium 2
Both the same
The greater the difference in the speed of light between the two media, the greater the bending of the light rays.Slide11
Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media?
1
3
2
1. n
1
> n
2
> n
3
2. n
3
> n
2
> n
1
3.
n
2
> n
3
> n14. n
1 > n3 > n25. none of the aboveSlide12
Parallel light rays cross interfaces from medium 1 into medium 2 and then into medium 3. What can we say about the relative sizes of the indices of refraction of these media?
1
3
2
1. n
1
> n
2
> n
3
2. n
3
> n
2
> n
1
3.
n
2
> n
3
> n14. n
1 > n3 > n25. none of the above Rays are bent toward the normal when crossing into #2, so n2 > n1. But rays are
bent away from the normal when going into #3, so n3 < n2. How to find the relationship between #1 and #3? Ignore medium #2! So the rays are bent away from the normal if they would pass from #1 directly into #3. Thus, we have: n
2
> n
1
> n
3
.Slide13
Apparent Depth
Light exits into medium (air) of lower index of refraction, and turns left. Slide14
Spear-FishingSpear-fishing is made more difficult by the bending of light.To spear the fish in the figure, one must aim at a spot in front of the apparent location of the fish.Slide15
n
2
n
1
d
d
Apparent depth:
Apparent Depth
50
actual fish
apparent fishSlide16
To spear a fish, should you aim directly at the image, slightly above, or slightly below?
1. aim directly at the image
2. aim slightly above3. aim slightly belowSlide17
To spear a fish, should you aim directly at the image, slightly above, or slightly below?
1. aim directly at the image
2. aim slightly above3. aim slightly below
Due to refraction, the image will appear
higher
than the actual fish, so you have to
aim
lower
to compensate.Slide18
To shoot a fish with a laser gun, should you aim directly at the image, slightly above, or slightly below?
1. aim directly at the image
2. aim slightly above3. aim slightly below
laser beam
light from fish
The
light
from the laser beam will also
bend
when it hits the air-water interface, so
aim
directly
at the fish
.Slide19
Delayed SunsetThe sun actually falls below below the horizon It "sets", a few seconds before we see it set. Slide20
Broken PencilSlide21
Water on the Road MirageSlide22
Palm Tree MirageSlide23
Mirage Near Dana – Home of Ernie PyleSlide24
Texas MirageSlide25Slide26
LoomingSlide27
Antarctic LoomingSlide28
LoomingSlide29
LoomingSlide30
Types of LensesSlide31
Lens TermsSlide32
Three Rays to Locate ImageRay parallel to axis bends through the focus.Ray through the focus bends parallel to axis.Ray through center of lens passes straight through.Slide33
Characterizing the ImageImages are characterized in the following wayVirtual or RealUpright or Inverted
Reduced, Enlarged, Same SizeSlide34
Object Beyond 2fImage isRealInvertedReducedSlide35
Object at 2f
Image isRealInvertedSame sizeSlide36
Object Between 2f and f
Image isRealInvertedEnlargedSlide37
Object at FNo Image is Formed!Slide38
Object Closer than FImage isVirtualUprightEnlargedSlide39
Converging Lens ImagesSlide40
Beacon Checkpoint
A beacon in a lighthouse is to produce a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed?Outside the focal pointAt the focal pointInside the focal pointSlide41
Lens in WaterCheckpoint
P.A.
F
Focal point determined by geometry and Snell’s Law:
n
1
sin(
q
1
) = n
2
sin(
q
2
)
Fat in middle = Converging
Thin in middle = Diverging
Larger n
2
/n1 = more bending, shorter focal length.n1 = n2 => No Bending, f = infinityLens in water has _________ focal length!
n1<n2Slide42
Lens in WaterCheckpoint
P.A.
F
Focal point determined by geometry and Snell’s Law:
n
1
sin(
q
1
) = n
2
sin(
q
2
)
Fat in middle = Converging
Thin in middle = Diverging
Larger n
2
/n1 = more bending, shorter focal length.n1 = n2 => No Bending, f = infinityLens in water has larger focal length!
n1<n2Slide43
Half Lens
Checkpoint
A converging lens is used to project a real image onto a screen. A piece of black tape is then placed over the upper half of the lens.
How much of the image appears on the screen?
1. Only the lower half will show on screen
2. Only the upper half will show on screen
3. The whole object will still show on screenSlide44
Half Lens
Checkpoint
A converging lens is used to project a real image onto a screen. A piece of black tape is then placed over the upper half of the lens. Slide45
Half Lens
Checkpoint
Still see entire image (but dimmer)! Slide46
Two very thin converging lenses each with a focal length of 20 cm are are placed in contact. What is the focal length of this compound lens?
10 cm
20 cm40 cmSlide47
Two very thin converging lenses each with a focal length of 20 cm are are placed in contact. What is the focal length of this compound lens?
10 cm
20 cm40 cmSlide48
Concave (Diverging) LensRay parallel to axis refracts as if it comes from the first focus.Ray which lines up with second focus refracts parallel to axis.Ray through center of lens doesn’t bend.Slide49
Image Formed by Concave LensImage is alwaysVirtualUprightReducedSlide50
Concave Lens Image DistanceAs object distance decreasesImage distance decreasesImage size increasesSlide51
Image CharacteristicsCONVEX LENS – IMAGE DEPENDS ON OBJECT POSITIONBeyond F: Real; Inverted; Enlarged, Reduced, or Same SizeCloser than F: Virtual, Upright, EnlargedAt F: NO IMAGE
CONCAVE LENS – IMAGE ALWAYS SAMEVirtualUprightReducedSlide52
Lens Equationsconvex: f > 0; concave: f < 0 do > 0 if object on left of lens di > 0 if image on right of lens otherwise di < 0 h
o & hi are positive if above principal axis; negative below
d
o
d
iSlide53
Which way should you move object so image is real and diminished?
Closer to the lensFarther from the lensA converging lens can’t create a real, diminished image.
F
F
Object
P.A.Slide54
Which way should you move object so image is real and diminished?
Closer to the lensFarther from the lensA converging lens can’t create a real, diminished image.
F
F
Object
P.A.Slide55
Image
Object
Image
Object
Object
Image
3 Cases for Converging Lenses
This could be used as a projector. Small slide on big screen
This is a magnifying glass
This could be used in a camera. Big object on small film
Upright
Enlarged
Virtual
Inverted
Enlarged
Real
Inverted
Reduced
Real
Inside F
Past 2F
Between
F & 2FSlide56
1)
Rays
parallel to principal axis pass through focal point.
2)
Rays through
center
of lens are not refracted.
3)
Rays
toward F
emerge parallel to principal axis.
Diverging Lens Principal Rays
F
F
Object
P.A.
Image is
(always true): Real or Imaginary
Upright or Inverted
Reduced or Enlarged
ExampleSlide57
1)
Rays
parallel to principal axis pass through focal point.
2)
Rays through
center
of lens are not refracted.
3)
Rays
toward F
emerge parallel to principal axis.
Diverging Lens Principal Rays
F
F
Object
P.A.
Image is
virtual, upright and reduced.
Example
ImageSlide58
Which way should you move the object to cause the image to be real?
Closer to the lensFarther from the lensDiverging lenses can’t form real images
F
F
Object
P.A.Slide59
Which way should you move the object to cause the image to be real?
Closer to the lensFarther from the lensDiverging lenses can’t form real images
F
F
Object
P.A.Slide60
Multiple Lenses
Image
from lens 1 becomes
object
for lens 2
1
f
1
f
2
2
Example
Complete the Rays to locate the final image.Slide61
Multiple Lenses
Image
from lens 1 becomes
object
for lens 2
1
f
1
f
2
2
ExampleSlide62
Multiple Lenses: Magnification
f1
f2do = 15 cm
f
1
= 10 cm
d
i
= 30 cm
f
2
= 5 cm
L = 42 cm
d
o
=12 cm
di = 8.6 cm
1
2
Example
Net magnification:
m
net
= m
1
m
2