Light, Reflection, and Refraction

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Light, Reflection, and Refraction

• Chapters 14 and 15

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Electromagnetic Waves

• An electromagnetic wave is composed of a magnetic
  field wave perpendicular to an electric field
  wave
• All objects that are not at absolute zero emit
  EMWs.
• The hotter the object the more waves they emit.
• The electromagnetic spectrum is composed of a
  range of wavelengths and frequencies that range
  from radio waves to gamma waves.
• Visible light is a very small portion of that
  entire spectrum.

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c

• The speed of an electromagnetic wave in a vacuum
  is 3.00 x 108m/s.
• It is equal to the product of the wavelength and
  the frequency
• c ?ƒ
• Sample Problem 14A

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Visible Light

• Visible Light is the part of the EMS that we can
  see
• Ranges from the color red with a wavelength of
  700nm (x10-9m) to the color purple with a
  wavelength of 400nm.

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Reflection

• Light waves usually travel in straight paths.
• When a light wave encounters a different
  substance it changes direction.
• When it encounters a substance that does not
  permit light to travel through it, opaque, some
  of the light will be reflected.
• Usually a portion of the light is absorbed.

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Reflection (cont)

• The texture of the opaque objects surface
  affects how it reflects light.
• A rough object reflects light in many different
  directions, diffuse reflection
• A smooth object reflects light in only one
  direction, specular reflection
• A surface is considered smooth if variations are
  smaller than the size of the wavelengths being
  reflected.
• It is difficult to make objects smooth enough to
  reflect X-rays and Gamma Rays.

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Mirrors

• Mirrors are smooth surfaces that reflect nearly
  all of the light they encounter.
• Light that strikes a mirror at an angle from the
  normal line reflects at the same angle away from
  the normal line

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Flat Mirrors

• Flat mirrors are the simplest form of mirror
  where the objects distance to the mirror, p, is
  equal to the distance from the mirror to the
  image, q.
• The image appears to be located behind the mirror
  and is considered to be a virtual image as the
  object would not appear on a screen.

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Ray Diagrams

• Ray diagrams are used to predict the location of
  the image of an object.
• To make a ray diagram for a flat mirror choose a
  point on the object and draw a ray toward the
  mirror at a perpendicular angle. This ray would
  reflect back on itself.
• Then draw a ray at an angle toward the mirror and
  draw the reflection of that ray.
• Trace back both of the reflected rays through the
  mirror, where they intersect, place the image.

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Concave Spherical Mirrors

• Concave spherical mirrors are those who
  reflective surface is on the interior of a curved
  surface that has a radius R to the center of
  curvature C.
• The optical axis is any line that passes through
  C and is usually oriented with an object.

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Concave Spherical Mirror Rules

• A ray traveling through C will reflect back
  through C. (only if object is beyond C)
• A ray traveling through the focal point f,
  halfway between C and the surface of the mirror,
  will reflect parallel to the OA
• A ray traveling to the intersection of the OA and
  the mirror will reflect at the same angle below
  the OA.
• A ray traveling parallel to OA will reflect
  through the focal point

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Ray Diagrams

• Using any of the two rules you must draw two
  rays, the object occurs at the point of
  intersection.
• We will draw several ray diagrams to determine
  the image produced by an object that is
• Beyond C
• Between C and f
• Between f and mirror

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Convex Spherical Mirrors

• Convex spherical mirrors are those where the
  reflective surface is on the outside of the
  curve.
• The points f and C are located behind the mirror
• Convex spherical mirrors have rules as well.

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Rules

• A ray parallel to the OA will reflect directly
  away from f.
• A ray heading towards f will reflect parallel to
  the OA
• A ray heading towards C will reflect directly
  away from C.
• A ray heading toward intersection of OA 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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Equations

• While ray tracing gives us a good idea of the
  location of an object it is always best to verify
  with math.
• If p is the objects distance and q is the image
  distance then
• 1/p 1/q 1/f
• The magnification of the object can been
  calculated using the equation
• M -(p/q)
• Sample Problem 14C

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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.
• When light passes from one transparent/translucent
  substance to another it changes direction.
• This change is due to the slight differences in
  speed that light travels in the new substance.
• This is called refraction.

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Analogy

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

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Index of Refraction

• The ratio of the speed of light in a vacuum to
  the speed of light in a medium is that mediums
  index of refraction. (n)
• The higher the index of refraction, the slower
  light travels through a medium.
• Refraction causes objects to appear at locations
  they are not at.

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Snells Law

• Snells Law relates the indices of refraction as
  well as the angle away from the normal line
  (angle of incidence) to determine the angle of
  refraction.
• n1(sin?i) n2(sin?r)
• ?r sin-1(n1/ n2)(sin?i)
• Sample Problem 15A

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Total Internal Reflection

• If the angle of incidence of a ray is very
  large(close to 90º) the ray will reflect rather
  than refract.
• This principal is responsible for the properties
  of fiber optic cables.
• Remember the lawn mower analogy

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Thin Lenses

• Refraction is the property that allows us to
  manipulate an objects image using a lens.
• We will be working with converging and diverging
  lenses.
• Just like with mirrors, we will need to follow
  rules to draw ray diagrams to predict the
  location of an image.
• Thin lenses also have focal points, these points
  are determined not only by the curve of the lens
  but the index of refraction of the lens as well.
  
• A lens has two focal points, one on either side.

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Converging Lens Diagram

• Draw one ray parallel to OA, refracts through
  focal point.
• Draw one ray through center of lens, continues
  straight through.
• Draw one ray through focal point, refracts
  through lens, travels parallel to OA.
• Image located at intersection of rays.
• 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 OA, refracts away from f,
  trace back to f.
• Ray 2 - ray toward f, refracts parallel to OA,
  trace back parallel to OA
• Ray 3 - ray through center, continues straight,
  trace back toward object

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Equations

• You can use the same equations for curved mirrors
  with lenses
• If p is the objects distance and q is the image
  distance then
• 1/p 1/q 1/f
• The magnification of the object can been
  calculated using the equation
• M -(p/q)
• Sample problem 15B