Light, Reflection, and Refraction

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Light, Reflection, and Refraction

• Chapters 14 and 15
• OPTICS

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Electromagnetic Waves

• Magnetic field wave perpendicular to an electric
  field wave
• All objects emit EMWs.
• ? Temp ?EMW
• Electromagnetic spectrum
• Range of all frequencies of light
• Visible light is a very small portion of that
  entire spectrum.

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c

• Speed of Light - 3.00 x 108m/s.
• (wavelength) x (frequency)
• c ?ƒ

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Example

• AM Radio waves
• 5.4 x 105 Hz
• 1.7 x 106 Hz
• ? ?

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Visible Light

• Part of the EMS humans can see
• Red - 750nm (x10-9m)
• Purple - 380nm
• Bees, Birds UV
• Snakes IR

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Reflection

• Light waves usually travel in straight paths
• Change in substance changes direction
• Opaque - does not permit light
• some light reflected
• some light absorbed as heat

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Reflection

• Texture affects reflection
• Diffuse reflection (rough)
• reflects light in many different directions,
• Specular reflection (smooth)
• reflects light in only one direction
• Smooth variations in surface ? ?

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Mirrors

• Light striking a mirror reflects at the same
  angle that it struck the mirror

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Flat Mirrors

• p q
• p- objects distance to the mirror
• q - distance from the mirror to the image
• Virtual image
• Does not exist
• Made by our eyes

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Ray Diagrams

• Used to predict the location of the image of an
  object

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Concave Spherical Mirrors

• Reflective surface is on the interior of a curved
  surface
• C center of curvature
• R Radius (distance to C)
• f Focal Point (1/2 R)
• Principal axis
• any line that passes through C
• usually oriented with an object

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Mirror Equations

• 1/object distance 1/image distance
  1/focal length
• 1/p 1/q 1/f
• Magnification (M)
• Image height/object height (h? / h)
• - (q / p)
• M h? / h - (q / p)

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Sign of Magnification
Sign of M Orientation of Image Type of Image
Upright Virtual
Inverted Real
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Concave Spherical Mirror Rules

• A ray traveling through C will reflect back
  through C
• A ray traveling through (f) will reflect parallel
  to the PA
• A ray traveling to the intersection of the PA and
  the mirror will reflect at the same angle below
  the PA.
• A ray traveling parallel to PA will reflect
  through the focal point

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Ray Diagrams

• Draw three rays
• The image forms at the point of intersection
• Example
• f 10.0cm
• p 30.0cm
• h 3.00cm

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Convex Spherical Mirrors

• Reflective surface is on the outside of the
  curve.
• The points f and C are located behind the mirror
• negative

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Rules

• A ray parallel to the PA will reflect directly
  away from f.
• A ray towards f will reflect parallel to the PA
• A ray towards C will reflect directly away from
  C.
• A ray to the intersection of PA and mirror will
  reflect at the same angle below the OA.
• Trace the 3 diverging lines back through the
  mirror to reveal the location of the image which
  is always virtual

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Example

• f -8.00cm
• p 10.0cm
• h 3cm

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Parabolic Mirrors

• Rays that hit spherical mirrors far away from the
  OA often reflect though other points causing
  fuzzy images, spherical aberration.
• Telescopes use parabolic mirrors as they ALWAYS
  focus the rays to a single point.

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Refraction

• Substances that are transparent or translucent
  allow light to pass though them.
• Changes direction of light
• Due to the differences in speed of light

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Analogy

• A good analogy for refracting light is a
  lawnmower traveling from the sidewalk onto mud

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Index of Refraction (n)

• The ratio of the speed of light in a vacuum to
  the speed of light in a medium
• ? n - ? c

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Snells Law

• ni(sin?i) nr(sin?r)
• ?r sin-1(ni/ nr)(sin?i)
• Example
• ?i 30.0
• ni 1.00
• nr 1.52

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?i 30.0 ni 1.00 nr 1.52
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Total Internal Reflection

• If the angle of incidence of a ray is greater
  than a certain critical angle the ray will
  reflect rather than reflect
• This principal is responsible for the properties
  of fiber optic cables.
• Remember the lawn mower analogy

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Critical Angle

• sin Tc nr / ni
• As long as nr lt ni
• What is the critical angle for light traveling
  from Diamond to Air?

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nr 1.000 ni 2.419
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Thin Lenses

• Converging
• Diverging
• f- curve of lens index of refraction

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Converging Lens Diagram

• Ray parallel to PA, refracts through far focal
  point
• Ray through center of lens, continues straight
  line
• Ray through near focal point, refracts through
  lens, continues parallel to PA
• Treat lens as though it were a flat plane.

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Diverging Lens Diagram

• Because the rays that enter a diverging lens do
  not intersect a virtual image is formed by
  tracing back the refracted rays.
• Ray 1 - parallel to PA, refracts away from near
  f, trace back to near f.
• Ray 2 - ray toward far f, refracts parallel to
  PA, trace back parallel to PA
• Ray 3 - ray through center, continues straight,
  trace back toward object

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Sign Conventions for Lens
Sign p q F
Near side of lens Far side of lens Converging Lens
Far side of lens Near side of lens Diverging Lens
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Converging Lens Example

• p 30.0cm
• f 10.cm

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Diverging Lens Example

• p 12.5cm
• f -10.0cm