The Unsorted BBS Collection/thegreatunsorted.tar/thegreatunsorted/texts/term_papers/hologram.esy

Text File | 1988-02-22 | 7KB | 110 lines

    Holograms

          Toss a pebble in a pond -see the  ripples?   Now  drop  two
    pebbles close together.  Look at what happens when the  two  sets
    of waves combine -you get a new wave!  When a crest and a  trough
    meet,  they cancel out and the water goes flat.  When two  crests
    meet, they produce one,  bigger crest.  When two troughs collide,
    they make a single,  deeper trough.  Believe it or  not,   you've
    just found a key to understanding how a hologram works.  But what
    do waves  in  a  pond  have  to  do  with  those  amazing  three-
    dimensional pictures?  How do waves make a hologram look like the
    real thing?

          It all starts with light.  Without it,  you can't see.  And
    much like the ripples in a pond,  light travels in  waves.   When
    you look at, say, an apple,  what you really see are the waves of
    light reflected from it.  Your  two  eyes  each  see  a  slightly
    different view of the apple.   These  different  views  tell  you
    about the apple's depth -its form and where it sits  in  relation
    to other objects.  Your brain processes this information so  that
    you see the apple,  and the rest of the world,  in 3-D.  You  can
    look around objects,  too -if the apple is blocking the  view  of
    an orange behind it,  you can just move your head  to  one  side.
    The apple seems to "move" out of the  way  so  you  can  see  the
    orange or even the back of  the  apple.   If  that  seems  a  bit
    obvious,   just  try  looking  behind  something  in  a   regular
    photograph!  You can't,  because the photograph  can't  reproduce
    the infinitely complicated waves of light reflected  by  objects;
    the lens of a camera can only focus those waves into a flat,  2-D
    image.  But a hologram can capture a 3-D image so  lifelike  that
    you can look around the image of the apple to an  orange  in  the
    background -and it's all thanks to  the  special  kind  of  light
    waves produced by a laser.

          "Normal" white light from the  sun  or  a  lightbulb  is  a
    combination of every colour of light in the spectrum -a  mush  of
    different waves that's useless for holograms.  But a laser shines
    light in a thin, intense beam that's just one colour.  That means
    laser light waves are uniform and in step.  When two laser  beams
    intersect,  like two sets of ripples meeting  in  a  pond,   they
    produce a single new wave pattern:  the hologram.  Here's how  it
    happens:  Light coming from a laser  is  split  into  two  beams,
    called the object beam and the reference beam.  Spread by  lenses
    and bounced off a mirror,  the object beam hits the apple.  Light
    waves reflect from the apple towards a  photographic  film.   The
    reference beam heads straight to the  film  without  hitting  the
    apple.  The two sets of waves meet and create a new wave  pattern
    that hits the film and exposes it.  On the film all you  can  see
    is a mass of dark and light  swirls  -it  doesn't  look  like  an
    apple at all!  But shine the laser  reference  beam  through  the
    film once more and the pattern of swirls bends the light  to  re-
    create the original reflection waves from the apple -exactly.

          Not all holograms work this way -some use plastics  instead
    of photographic film,  others are visible in normal  light.   But
    all holograms are created with lasers -and new waves.

    All Thought Up and No Place to Go

          Holograms were invented  in  1947  by  Hungarian  scientist
    Dennis Gabor,  but they were ignored for years.  Why?  Like  many
    great ideas,  Gabor's theory about light waves was ahead  of  its
    time.  The lasers needed to produce clean waves -and  thus  clean
    3-D images -weren't invented until 1960.  Gabor coined  the  name
    for his photographic technique from holos and gramma,  Greek  for
    "the whole message. " But for more than a decade,  Gabor had only
    half the words.  Gabor's contribution to science  was  recognized
    at last in 1971 with a Nobel Prize.  He's got a chance for a last
    laugh, too.  A perfect holographic portrait of the late scientist
    looking up from his  desk  with  a  smile  could  go  on  fooling
    viewers into saying hello forever.  Actor  Laurence  Olivier  has
    also achieved that kind of immortality  -a  hologram  of  the  80
    year-old can be seen these days on the stage  in  London,   in  a
    musical called Time.

    New Waves

          When it comes to looking at the future uses of  holography,
    pictures are anything but the whole picture.   Here  are  just  a
    couple of the more unusual possibilities.  Consider this:  you're
    in a windowless room in the  middle  of  an  office  tower,   but
    you're reading by the light of the noonday sun!  How can this be?
    A new invention that incorporates holograms into  widow  glazings
    makes it possible.  Holograms can bend light to create complex 3-
    D images,  but they can also simply  redirect  light  rays.   The
    window glaze holograms could  focus  sunlight  coming  through  a
    window into a narrow beam,  funnel  it  into  an  air  duct  with
    reflective walls above the ceiling and send it down the  hall  to
    your windowless cubbyhole.  That could  cut  lighting  costs  and
    conserve energy.  The holograms could even  guide  sunlight  into
    the gloomy gaps between city skyscrapers and since they can  bend
    light of different colors in different directions,  they could be
    used to filter out  the  hot  infrared  light  rays  that  stream
    through your car windows to bake you on summer days.

          Or,  how about holding an entire library  in  the  palm  of
    your hand?  Holography makes it theoretically possible.  Words or
    pictures could be translated into a  code  of  alternating  light
    and dark spots and stored in an unbelievably tiny space.   That's
    because light waves are very,  very skinny.  You could lay  about
    1000 lightwaves side by side across the width of  the  period  at
    the end of this sentence.  One calculation holds  that  by  using
    holograms,  the U. S.  Library of Congress could be stored in the
    space of a sugar cube.  For now, holographic data storage remains
    little more than a fascinating idea because the materials  needed
    to do the job haven't been invented yet.   But  it's  clear  that
    holograms,   which  author  Isaac  Asimov  called  "the  greatest
    advance in imaging since the eye" will continue to make waves  in
    the world of science.