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Lenses

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					           Lenses

By Micah Massari and Dean Yoder
       Converging and Diverging
               Lenses
• A lens is a piece of glass that can bend parallel rays of light
  so they cross or appear to have crossed.
• Lenses can cause light to converge to a single point or
  diverge from a single point.
• A converging lens is thicker in the middle than on the
  outsides and a diverging lens is thinner in the middle then
  on the outside.
• Principal axis of a lens is the line joining the centers of
  curvatures of its surface.
• Focal point is the point at which light converges when
  beamed parallel to the principal axis. A focal plane is made
  up of beams that focus above or below the focal plane.
• Focal length the distance between the center of the lens
  and its focal point.
     Image Formation by a Lens
• Far away objects are seen through a small angle of view
  while closer objects are seen through a larger angle of
  view. The wider angle allows you to see more detail.
• Converging lenses help magnify or let you look at
  something through a wider angle.
• Magnifying glasses can only magnify an object within its
  focal length.
• When an object is farther away than the focal point of a
  converging lens light does converge and it can be focused
  on a screen as a real image.
• Diverging lenses are used as view finders for cameras the
  virtual image you see is right side up and smaller than the
  object.
    Constructing Images Through
           Ray Diagrams
•   Ray diagrams are used to determine the location and
    size of an image. The image can be located by following
    light ray paths, there are three:
      1.   The first ray starts parallel to the principal axis and then is refracted
           to the focal point.
      2.   The next path is through the center of the lens and continue in a
           strait line.
      3.   The third path is when a ray passes through the focal point of the
           lens and emerges parallel to the principal axis.
–   Only two of these rays are needed to find locate an
    image.
–   Ray diagrams can also be used for diverging lenses.
   Image Formation Summarized
• When an object is within one focal length it is a simple
  magnifying glass that produces a magnified , right side up,
  virtual image. When an object is beyond one focal length it
  produces a real inverted image.
• When an object is viewed with a diverging lens the image
  is virtual, reduced, and right side up.
    Common Optical Instruments
• The camera- a lens and sensitive film are mounted in a light
  tight box. The lens forms a real inverted image on the film.
• The telescope- a lens forms a real image of a distant object
  the image is projected into space to be viewed by another
  magnifying like lens.
• Binoculars- are like two telescopes side by side except they
  have prisms inside to flip images right side up. They are
  less bright than a normal telescope with two lenses because
  the light has to pass through all of the prisms making them
  less bright.
   Common Optical Instruments
• Compound microscope- uses two converging lenses with
  short focal length. The objective lens is the first lens; it
  produces a real image of a close up object. The eye piece is
  the second lens and it forms a virtual image that is even
  more enlarged.
• Projector- a concave mirror reflects light from an intense
  light source onto a pair of condenser lenses; these direct
  the light through the slide to a projection lens. The
  projection lens is mounted so it can be moved back and
  forth for focusing.
                        The Eye
• Iris- regulates the amount of light that can enter your eye
  and it also surrounds the pupil.Light enters through the
  cornea then it passes through the pupil and lens and is
  focused on the retina.
• The fovea is the most sensitive part of the retina and allows
  the view of greater detail.
• You see images upside down but your brain has learned to
  flip it.
• To focus an image your eyes accommodate by thickening or
  thinning your lens by using ciliary muscles that surround the
  lens.
• Vision is sharpest in the eye when the pupil is the smallest.
  Some Defects in Vision and in
            Lenses
• People with normal vision can see objects from infinity
  down to about 25 cm.
• Farsighted- people with this problem form images behind
  their retina so they must hold objects more than 25 cm
  away to see it clearly. To fix this the person may wear
  glasses with converging lenses.
• Nearsighted- the person can see objects nearby very
  clearly but far away objects are out of focus. This is
  because they form images in front of the retina. This
  problem is fixed by wearing glasses with diverging lenses.
   Some Defects in Vision and in
             Lenses
• Astigmatism- this is caused when one part of the cornea
  is bent more than the other. For this cylindrical glasses are
  prescribed.
• No lens can give a perfect image combining lenses can
  reduce aberrations though making clearer images.
• Spherical aberration is caused when light rays that pass
  through a lens focus at different points. This is corrected
  by adding a diaphragm to the outside or combining other
  lenses.
• Chromatic aberration is caused by the different speeds of
  different colors of light which causes different refractions.
  This is fixed by using Achromatic lenses which are lenses
  combined to correct this effect.

				
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