Thin Lenses

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Thin Lenses
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Any lens that is thicker in the center than at
the edges will make parallel rays converge to a
point and is called a converging lens. Lenses
that are thinner in the center than at the edges
are called diverging lenses because they make
parallel light diverge.
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Finding the Image Position Formed by a Thin Lens
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• ray 1 is drawn parallel to the axis therefore it
  is refracted by the lens so that is passes along
  a line through the focal point
• ray 2 is drawn on a line passing the other focal
  point F and emerges from the parallel to the
  axis
• ray 3 is directed toward the very center of the
  lens, this ray emerges from the lens at the same
  angle as it entered.

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The Lens Equation
Converging Lens
Diverging Lens
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1. The focal length is positive for converging lens
   and negative for diverging.
2. The object distance is positive if it is on the
   same side of the lens from where the light is
   coming otherwise it is negative.
3. The image distance is positive if it is on the
   opposite side of the lens from where light is
   coming.
4. The height of the image is positive if the image
   is upright and negative if the image is inverted
   relative to the object.

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Lateral Magnification
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1. You find a diverging (converging) lens in the
   storeroom and wish to define its focal lens.
   Describe how you could use two lasers to
   accomplish this.
2. What type of lens would you use to construct an
   overhead projector? Explain your reasoning.
3. Two converging lenses with identical shapes are
   made from glasses with different indices of
   refraction. Which one has the shorter focal
   length? Why?
4. What type of lens would be helpful in starting of
   a campfire? Why?

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Optical Instruments
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Pinhole Cameras
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Cameras
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Our Eyes
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Normal Eye
Lens relaxed, focused at infinity Lens
thickened, focused on nearby object
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Correcting eye defects with lenses
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Test for Astigmatism
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The Magnifying Glass
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Telescopes
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Newtonian
Cassegrainian

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1. In a refracting telescope, parallel light is
   collected from a distant source to form a real
   image that is then magnified by the eyepiece. To
   achieve high magnification, this real image in
   located near the eyepieces focal point. Use this
   information to determine how the length of a
   refracting telescope compares with the local
   lengths of the lenses.
2. Sometimes when a person develops a cataract, the
   lens of the eye is surgically removed and
   replaced with a plastic one. Would you expect
   this lens to be converging or diverging one?
   Explain.

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Baby-Quiz

1. Why is the diamond more brilliant than a clear
   piece of glass having the same shape?
2. A surface appear yellow under the white light.
   How it will appear under red light? Under green
   light? Under blue light?
3. Most of us find that we really have to strain our
   eyes to focus on objects located close to our
   noses. You hold two mirrors 1 foot in front of
   your face. One is a plane mirror, and the other
   is a concave mirror with a 3-inch focal length.
   In which case are you more likely to have to
   strain your eyes to see the image of your nose?
4. When you mix red and green light from separate
   projectors, you get yellow spot on the wall.
   However, if you mix red and green paint, you get
   a muddy brown color. How do you account for this
   difference?