Images

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Images
Chapter
34
One of the most important uses of the basic
laws governing light is the production of images.
Images are critical to a variety of fields and
industries ranging from entertainment, security,
and medicine In this chapter we define and
classify images, and then classify several basic
ways in which they can be produced.
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The Sun is about 1.5 1011 m away. The time for
light to travel this distance is about A. 4.5
1018 s B. 8 s C. 8min D. 8 hr E. 8 yr
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Two Types of Images
real image
lens
object
object
mirror
virtual image
Image a reproduction derived from light Real
Image light rays actually pass through image,
really exists in space (or on a screen for
example) whether you are looking or not Virtual
Image no light rays actually pass through image.
Only appear to be coming from image. Image only
exists when rays are traced back to perceived
location of source
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A Common Mirage
Light travels faster through warm air ? warmer
air has smaller index of refraction than colder
air ? refraction of light near hot surfaces For
observer in car, light appears to be coming from
the road top ahead, but is really coming from sky.
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Plane Mirrors, Point Object
Plane mirror is a flat reflecting surface.
Identical triangles
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Since I is a virtual image i lt 0
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Plane Mirrors, Extended Object
Each point source of light in the extended object
is mapped to a point in the image
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Plane Mirrors, Mirror Maze
Your eye traces incoming rays straight back, and
cannot know that the rays may have actually been
reflected many times
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Spherical Mirrors, Making a Spherical Mirror
plane
Plane mirror ? Concave Mirror 1. Center of
Curvature C in front at infinity ? in front
but closer 2. Field of view wide ? smaller 3.
Image ip ? igtp 4. Image height image
height object height ? image height gt object
height
concave
Plane mirror ? Convex Mirror 1. Center of
Curvature C in front at infinity ? behind
mirror and closer 2. Field of view wide ?
larger 3. Image ip ? iltp 4. Image
height image height object height ? image
height lt object height
convex
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Spherical Mirrors, Focal Points of Spherical
Mirrors
convex
concave
r gt 0 for concave (real focal point) r lt 0 for
convex (virtual focal point)
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Images from Spherical Mirrors
Start with rays leaving a point on object, where
they intersect, or appear to intersect marks the
corresponding point on the image.
Real images form on the side where the object is
located (side to which light is going). Virtual
images form on the opposite side.
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Locating Images by Drawing Rays

1. A ray parallel to central axis reflects through F
2. A ray that reflects from mirror after passing
   through F, emerges parallel to central axis
3. A ray that reflects from mirror after passing
   through C, returns along itself
4. A ray that reflects from mirror after passing
   through c is reflected symmetrically about the
   central axis

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Proof of the magnification equation
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Spherical Refracting Surfaces
Real images form on the side of a refracting
surface that is opposite the object (side to
which light is going). Virtual images form on the
same side as the object.
When object faces a convex refracting surface r
is positive. When it faces a concave surface, r
is negative. CAUTION Reverse of of mirror sign
convention!
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Thin Lenses
Converging lens
Diverging lens
Lens only can function if the index of the lens
is different than that of its surrounding medium
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Images from Thin Lenses
Real images form on the side of a lens that is
opposite the object (side to which light is
going). Virtual images form on the same side as
the object.
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Locating Images of Extended Objects by Drawing
Rays

1. A ray initially parallel to central axis will
   pass through F2
2. A ray that initially passes through F1, will
   emerge parallel to central axis
3. A ray that initially is directed toward the
   center of the lens will emerge from the lens with
   no change in its direction (the two sides of the
   lens at the center are almost parallel)

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Two Lens System
i1
p2
i2
O
I2
p1
I1
O2
Lens 1
Lens 2

• Let p1 be the distance of object O from Lens 1.
  Use equation and/or principle rays to determine
  the distance to the image of Lens 1, i1.
• Ignore Lens 1, and use I1 as the object O2. If O2
  is located beyond Lens 2, then use a negative
  object distance p1. Determine i2 using the
  equation and/or principle rays to locate the
  final image I2.

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18
The time for a radar signal to travel to the Moon
and back, a one-way distance of about 3.8 108
m, is A. 1.3 s B. 2.5 s C. 8 s D. 8min E.
1 106 s
Radio waves of wavelength 3 cm have a frequency
of A. 1MHz B. 9MHz C. 100MHz D. 10, 000MHz E.
900MHz
The light intensity 10m from a point source is
1000W/m2. The intensity 100m from the same source
is A. 1000W/m2 B. 100W/m2 C. 10W/m2 D.
1W/m2 E. 0.1W/m2
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Light of uniform intensity shines perpendicularly
on a totally absorbing surface, fully
illuminating the surface. If the area of the
surface is decreased A. the radiation pressure
increases and the radiation force increases B.
the radiation pressure increases and the
radiation force decreases C. the radiation
pressure stays the same and the radiation force
increases D. the radiation pressure stays the
same and the radiation force decreases E. the
radiation pressure decreases and the radiation
force decreases
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A clear sheet of polaroid is placed on top of a
similar sheet so that their polarizing axes make
an angle of 30? with each other. The ratio of the
intensity of emerging light to incident
unpolarized light is A. 1/4 B. 1/3 C. 1/2 D.
3 /4 E. 3 /8
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Optical Instruments, Simple Magnifying Lens
Can make an object appear larger (greater angular
magnification) by simply bringing it closer to
your eye. However, the eye cannot focus on
objects closer that the near point pn25 cm?BIG
BLURRY IMAGE A simple magnifying lens allows the
object to be placed close by making a large
virtual image that is far away.
Object at F1
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Optical Instruments, Compound Microscope
O close to F1
I close to F1
Mag. Lens
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Optical Instruments, Refracting Telescope
I close to F2 and F1
Mag. Lens
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Three Proofs, The Spherical Mirror Formula
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Three Proofs, The Refracting Surface Formula
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Three Proofs, The Thin Lens Formulas
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