I. Parabolic Reflector Lab Background

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I. Parabolic Reflector Lab Background

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A. Light at Boundaries

• 1. The Law of Reflection
• The diagram below illustrates the law of
  reflection.

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• The law of reflection states that when a ray of
  light reflects off a surface, the angle of
  incidence is equal to the angle of reflection.

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2 (a) The ray of light approaching the mirror is
known as the incident ray. (b) The ray of
light which leaves the mirror is known as the
reflected ray (labeled R in the diagram). (c)
A line can be drawn perpendicular to the surface
of the mirror this line is known as a normal
line. (d) The angle between the incident ray
and the normal is known as the angle of
incidence. (e) The angle between the reflected
ray and the normal is known as the angle of
reflection.
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B. Parabolic Reflector

1. A parabolic reflector has a concave mirror that
   reflects light from its inner (caved in)
   surface.
2. Parabolic reflectors used as solar cookers take
   the parallel beams of light that come from the
   sun and focus them at one point called the focal
   point.

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• 3. A parallel beam is one which does not diverge
  or converge - its width stays the same as the
  light wave travels out from its source. The sun
  produces parallel beams.
• 4. A parabola has many uses within everyday life
  because it has the property of focusing to a
  point incident rays parallel to the axis.
  Automotive headlamps, microwave / radar antenna
  and most reflecting telescopes use parabolic
  surfaces in their designs to name a few.

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C. Curved Mirror Terminology

• 1. Center of curvature (C) is the center of the
  sphere from which a spherical mirror is taken.
• 2. Vertex (V) is the center of the mirror.
• 3. Principal axis is the radius connecting the
  center of curvature of a curved mirror with its
  geometric vertex.

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• 4. Normal is the line drawn perpendicular to the
  mirror surface at the incidence point.
• 5. Principal focus (F) is where the light rays
  converge to or diverge from.
• 6. Focal length ( f ) is the distance from the
  vertex to its principal focus.
• 7. The aperture determines the amount of light
  intercepted by the mirror.

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• 8. Incident light rays (or incoming light rays)
  are parallel to the principal axis and are
  reflected and converge at the principal focus.
  (This is where you will do your cooking.)

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D. Parabolic Equation

• 1. The graphs of quadratic functions have the
  characteristic shape of a parabola, where y ax2
  bx c , and the parabola can be mathematically
  defined as y x2 / 4f , where "x" is the
  radius of a dish and "f" is the focal length.
• 2. See diagram.

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II. Construction

• A. After you have completed your graph, you will
  need to take the graph and cut out the shape of
  the parabola that you will use as your pattern
  for the construction of the spines.
• B. The pattern will then be laid down on the
  material you wish to make the solar cooker from,
  such as insulation board.

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• C. Lay the pattern down and cut out the spines.
• Note to keep the shape of the parabola and not
  have any sag in the shape, you may need at least
  12 spines. (More would be better.)
• D. Once the spines are cut out, use a base board
  (made out of cardboard, wood, insulation board,
  etc.) to set the spines up in a circle.
• It may help to draw a small circle no more than 1
  inch (1 inch is 2.54 centimeters) in diameter in
  the center of the baseboard (use a small lid).
  Then set the spines up at the edge of this circle
  to keep everything uniform.

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Baseboard
The circle doesnt need to be more than around
one inch in diameter
Parabolic spines set up at edge of circle
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Note how the spines are set up
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• E. The spines can be hot glued, taped, or
  sandwiched in place by gluing blocks of
  material on either side of the spines to hold
  them in place.

Spine
Wedge
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• F. Once the spines are set up, you need to get
  some poster board (or possible cardboard) and cut
  it into pie slices).
• These need to be a little larger than the size of
  the spacing of the spines on the baseboard.
• G. Find the reflective surface you are going to
  use and glue it onto the pie slices. Make sure
  you smooth as many wrinkles out of the material
  as you can.
• H. Next glue or tape these to the spines.
• I. Finally place a hotdog holder onto the
  project to hold the hotdog securely in place.
  Make sure this is longer than the focal point of
  the cooker so that the hot dog can be slid down
  in place.

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