Lenses

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					Lenses


Chapter 30
          Converging and Diverging Lenses
   Lens – a piece of glass which bends parallel rays so
    that they cross and form an image
   Converging Lens – a lens which is thicker in the
    middle, makes parallel light rays converge
   Diverging Lens – a lens which is thinner in the
    middle, makes parallel light rays diverge
   Principle Axis – line joining the centers of curvature
    of a lens’ surface
   Focal Point – the point at which a beam of light
    converges
   Incident parallel beams that are not parallel to the
    principle axis focus at points above or below the focal
    point
   Focal Plane – all possible points from incident
    beams
   A lens has two focal points and two focal planes
Converging and Diverging Lenses
          Image Formation by a Lens
   Magnification occurs when an image is observed
    through a larger angle with the use of a lens than
    with out the lens, and allows more detail to be seen
   A converging lens will only magnify when the object
    is between the focal point and the lens
   Virtual Image – an image formed through reflection
    or refraction that can be seen by an observer but
    cannot be projected on a screen because light from
    the object does not actually come to a focus
   Real Image – an image that is formed by
    converging light rays and that can be displayed on a
    screen
   When a diverging lens is used alone, the image is
    always virtual, right-side up, and smaller than the
    object
Image Formation by a Lens
Constructing Images Through Ray
Diagrams
    Ray Diagrams – show the principle
     rays that can be used to determine that
     size and location of an image
    One end of the object is always placed
     right on the principle axis
    3 useful rays in ray diagrams
1.   Ray parallel to the principle axis that
     passes through the focal point after
     refraction by the lens
2.   A ray through the center of the lens that
     does not change direction
3.   A ray through the focal point in front of
     the lens that emerges parallel to the
     principle axis after refraction by the lens
Constructing Images Through Ray
Diagrams
Image Formation Summarized
   If an object is one focal length away from
    a converging lens, the image is virtual,
    magnified, and right-side up
   If an object is beyond one focal length, a
    converging lens produces a real, inverted
    image
   If the object is close to the focal point,
    the image appears far away; if it is far
    from the focal point, the image appears
    nearer
   When an object is viewed through a
    diverging lens, the image is virtual,
    reduced, and right-side up
      Some Common Optical Instruments

   The Camera – consists of a lens and sensitive
    film mounted in a lighttight box, the amount of
    light which gets to the film is regulated by a
    shutter and a diaphragm (which varies the size of
    the opening)
   The Telescope – a simple telescope uses a lens to
    form a real image of a distant object
   The Compound Microscope – uses two converging
    lenses of short focal length, forming a real and a
    virtual image (so we see the object right-side up)
   The Projector – An arrangement of converging
    lenses is used
Some Common Optical Instruments
The Eye
   The amount of light that enters is
    regulated by the iris, the colored part of
    the eye that surrounds the opening called
    the pupil
   Light enters through the transparent
    covering called the cornea, passes
    through the pupil and lens, and is focused
    on a layer of tissue at the back of the
    eye—the retina—extremely sensitive to
    light
   In both the camera and the eye, the
    image is upside down, our brain flips the
    image right-side up for us
The Eye
Some Defects in Vision

   Farsighted – person’s eyes form
    images behind the retina, the
    eyeball is too short
   Nearsighted – person’s eyes form
    images in front of the retina, the
    eyeball is too long
   Astigmatism – the cornea is
    curved more in one direction than
    the other, the eye does not form
    sharp images
Some Defects in Vision
Some Defects in Lenses
   Aberrations – the distortions in an
    image
   By combining lenses in certain ways,
    aberrations can be minimized
   Spherical aberration results when light
    passes through the edges of a lens and
    focuses at a slightly different place from
    light passing through the center of the
    lens
   Chromatic aberration is the result of the
    different speeds of light of various colors
    and hence the different refractions they
    undergo
Some Defects in Lenses

				
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posted:6/25/2011
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