Chapter 29

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Chapter 29 Reflection Refraction

• Chapter preview
• Sections
• Reflection
• The Law of Reflection
• Mirrors
• Diffuse Reflection
• Reflection of Sound
• Refraction
• Refraction of Sound
• Refraction of Light
• Atmospheric Refraction
• Dispersion in a Prism
• The Rainbow
• Total Internal Reflection

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Reflection Refraction

• Section 29.1Reflection

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Reflection

• Reflection some or all of a wave bounces back
  into the first medium when hitting a boundary of
  a second medium
• When all the wave energy is reflected back
  instead of being transmitted, it is total
  reflection
• If some energy is transmitted and some is
  reflected, the wave is partially reflected

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Reflection
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Reflection Refraction

• Section 29.2The Law of Reflection

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The Law of Reflection

• The direction of incidence and reflection is best
  described by straight-line rays
• Incident rays and reflected rays make equal
  angles with a line perpendicular to the surface,
  called the normal
• Angle of Incidence angle made by the incident
  ray and the normal
• Angle of Reflection angle made by the reflected
  ray and the normal
• Law of Reflection the angle of incidence and
  the angle of reflection are equal

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The Law of Reflection
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Reflection Refraction

• Section 29.3Mirrors

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Mirrors

• Virtual Image the point located behind a mirror
  where an object appears to originate
• Your eye cannot tell the difference between an
  object and its virtual image
• The image is as far behind a mirror as the object
  is in front of the mirror

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Mirrors

• The law of reflection holds for curved mirrors.
• a. The image formed by a Convex mirror is smaller
  than the object.
• b. When an object is close to a concave mirror,
  the image can be larger than the object.

• For reflections in a plane mirror, object size
  equals image size and object distance equals
  image distance.

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Reflection Refraction

• Section 29.4Diffuse Reflection

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Diffuse Reflection

• Diffuse Reflection light incident on a rough
  surface is reflected in many directions
• A surfaces roughness is dependent upon the
  wavelength of the wave incident upon that
  surface the longer the wavelength, the smoother
  the surface will appear
• To a piece of paper, light is reflecting
  diffusely

The Law of Reflection is Always Observed
(regardless of the orientation of the surface)
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Reflection Refraction

• Section 29.5Reflection of Sound

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Reflection of Sound

• An echo is reflected sound
• Sound reflects from all surfaces of a room
• Acoustics is the study of the way sound reflects
  off of objects in a room
• Reverberations Multiple reflections of sound
  within a room
• The walls of concert halls are designed to make
  the reflection of sound diffuse

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Reflection of Sound
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Reflection Refraction

• Section 29.6Refraction

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Refraction

• Refraction the change in direction of a wave as
  it crosses the boundary between two media in
  which the wave travels at different speeds
• Wave Fronts lines that represent the position
  of different crests
• At each point along a wave front, the wave is
  moving perpendicular to the wave front
• The direction of motion is best represented by a
  ray

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Refraction

• Incident Ray
• Refracted Ray

Less Rigid Medium _________________ More
Rigid Medium Refracted ray bends toward the
normal
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Refraction

• When one medium ends and another begins, that is
  called a boundary.
• When a wave encounters a boundary that is more
  dense, part of it is reflected and part of it is
  transmitted.
• The frequency of the wave is not altered when
  crossing a barrier, but the speed and wavelength
  are.
• The change in speed and wavelength can cause the
  wave to bend, if it hits the boundary at an angle
  other than 90.

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Reflection Refraction

• Section 29.7Refraction of Sound

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Refraction of Sound

• Sound waves are refracted when parts of a wave
  front travel at different speeds
• This happens in uneven winds or temperatures
• Sound waves tend to bend away from warm ground,
  since it travels faster in warmer air
• On a cold night, the speed of sound is slower
  near the ground than above, so we can hear over
  larger distances

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Refraction of Sound
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Reflection Refraction

• Section 29.8Refraction of Light

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Refraction of Light

• A pond or swimming pool may appear shallower than
  they actually are, a pencil in a glass of water
  will appear bent
• All of these effects are caused by changes in the
  speed of light as it passes from one medium to
  another, or through varying temperatures and
  densities of the same medium which changes the
  directions of light rays
• Index of Refraction (n) (speed of light in
  vacuum)/(speed of light in material)
• Snells Law n sin ? n sin ? (where n and n
  are the indices of refraction of the media on
  either side of the boundary, and ? and ? are the
  respective angles of incidence and refraction)
• Index of Refraction of a few substances
• Vacuum 1.00 Crown glass 1.52
• Air 1.0003 Quartz 1.52
• Water 1.33 Diamond 2.42
• Ethanol 1.36

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Refraction of Light
The submerged object's apparent depth equals its
true depth divided by the liquid's index of
refraction d' d(n2/n1). Note n2 is the index
of refraction of the medium above the surface and
n1 is the index of refraction of the medium below
the surface.

• Incident Ray
• Refracted Ray

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Refraction of Light

• Figure 29.19
• There are many effects of refraction
• a. The apparent depth of the glass block is less
  than the real depth.
• b. The fish appears to be nearer than it
  actually is.
• c. The full glass mug appears to hold more root
  beer than it actually does.

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Reflection Refraction

• Section 29.9Atmospheric Refraction

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Atmospheric Refraction
On hot days there may be a layer of very hot air
in contact with the ground, the light will travel
faster through this air and will bend, creating a
mirage
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Atmospheric Refraction

• When you watch the sun set, you can still see the
  sun for several minutes after it has sunk below
  the horizon, because light is refracted by
  Earths atmosphere

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Reflection Refraction

• Section 29.10Dispersion in a Prism

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Dispersion in a Prism

• Light of frequencies closer to the natural
  frequency of the electron oscillators in a medium
  travels more slowly in the medium
• Since different frequencies of light travel at
  different speeds in transparent materials, they
  will refract differently and bend at different
  angles
• When light is bent twice at nonparallel
  boundaries, as in a prism, the seperation of the
  different colors is apparent
• Dispersion the separation of light into colors
  arranged according to their frequency

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Dispersion in a Prism
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Reflection Refraction

• Section 29.11The Rainbow

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The Rainbow

• The rainbow takes the concept of dispersion and
  multiples it through the atmosphere
• The sun shines on water droplets in a cloud or
  when it is raining
• The light is dispersed by the raindrop into its
  spectral colors

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The Rainbow

• Dispersion by a Raindrop
• Each droplet acts like a prism
• Higher drops red is bent to the eye
• Lower drops violet is bent to the eye

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The Rainbow

• Rainbows will always appear at an angle between
  40? and 42? . If you are lucky enough to see two
  rainbows at the same time, the second (much
  dimmer) one occurs above the main one, and
  results from sunlight creating a double
  reflection in the water droplets. The colors are
  also upside down.

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Reflection Refraction

• Section 29.12Total Internal Reflection.

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Total Internal Reflection

• Critical Angle
• Figure 29.32
• You can observe total internal reflection in your
  bathtub.
• a-d) Light emitted in the water at angles below
  the critical angle is partly refracted and partly
  reflected at the surface.
• e) At the critical angle, the emerging beam
  shims the surface.
• f) Past the critical angle, there is total
  internal reflection.

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Total Internal Reflection

• Total Internal Reflection in Diamonds
• The critical angle for a diamond is 24.6?,
  smaller than in other common substances. This
  small critical angle means that light inside is
  more likely to totally internally reflect.

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Total Internal Reflection

• Critical Angle the minimum angle of incidence
  for which a light ray is totally reflected within
  a medium
• Total Internal Reflection the 100 reflection
  of light that strikes the boundary between two
  media at an angle greater than the critical angle
• Optical fibers utilize the concept of total
  internal reflection to feed light from one
  location to another, these cables are very useful
  for communications

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Total Internal Reflection

• Optical Fibers
• At each contact w/ the glass air interface, if
  the light hits at greater than the critical
  angle, it undergoes total internal reflection and
  stays in the fiber.