Conceptual Physics

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Conceptual Physics

• Chapter Twenty Seven Notes
• LIGHT .

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Introduction

• To understand light you have to know that what we
  call light is what is visible to us. Visible
  light is the light that humans can see. Other
  animals can see different types of light. Dogs
  can see only shades of gray and some insects can
  see light from the ultraviolet part of the
  spectrum. The key thing to remember is that our
  light is what scientists call visible light.
• Scientists also call light electromagnetic
  radiation. Visible light is only one small
  portion of a family of waves called
  electromagnetic (EM) radiation. The entire
  spectrum of these EM waves includes radio waves,
  which have very long wavelengths and both gamma
  rays and cosmic rays, which are at the other end
  of the spectrum and have very small wavelengths.
  Visible light is near the middle of the spectrum.
  
• The key thing to remember is that light and EM
  radiation carry energy. The quantum theory
  suggests that light consists of very small
  bundles of energy/particles it's just that
  simple. Scientists call those small particles
  photons, and the wavelength determines the energy.

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27.1 Early Concepts of Light

• For as long as the human imagination has sought
  to make meaning of the world, we have recognized
  light as essential to our existence. Whether to a
  prehistoric child warming herself by the light of
  a fire in a cave, or to a modern child afraid to
  go to sleep without the lights on, light has
  always given comfort and reassurance.
• The earliest documented theories of light came
  from the ancient Greeks. Aristotle believed that
  light was some kind of disturbance in the air,
  one of his four "elements" that composed matter.
  Centuries later, Lucretius, who, like Democritus
  before him, believed that matter consisted of
  indivisible "atoms," thought that light must be a
  particle given off by the sun. In the tenth
  century A.D., the Persian mathematician Alhazen
  developed a theory that all objects radiate their
  own light. Alhazens theory was contrary to
  earlier theories proposing that we could see
  because our eyes emitted light to illuminate the
  objects around us.
• In the seventeenth century, two distinct models
  emerged from France to explain the phenomenon of
  light. The French philosopher and mathematician
  Rene Descartes believed that an invisible
  substance,

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• which he called the plenum, permeated the
  universe. Much like Aristotle, he believed that
  light was a disturbance that traveled through the
  plenum, like a wave that travels through water.
  Pierre Gassendi, a contemporary of Descartes,
  challenged this theory, asserting that light was
  made up of discrete particles.
• Particles versus Waves
• While this controversy developed between rival
  French philosophers, two of the leading English
  scientists of the seventeenth century took up the
  particles-versus-waves battle. Isaac Newton,
  after seriously considering both models,
  ultimately decided that light was made up of
  particles (though he called them corpuscles).
  Robert Hooke, already a rival of Newtons and the
  scientist who would identify and name the cell in
  1655, was a proponent of the wave theory. Unlike
  many before them, these two scientists based
  their theories on observations of lights
  behaviors reflection and refraction. Reflection,
  as from a mirror, was a well-known occurrence,
  but refraction, the now familiar phenomenon by
  which an object partially submerged in water
  appears to be broken, was not well understood
  at the time.

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• Proponents of the particle theory of light
  pointed to reflection as evidence that light
  consists of individual particles that bounce off
  of objects, much like billiard balls. Newton
  believed that refraction could be explained by
  his laws of motion, with particles of light as
  the objects in motion. As light particles
  approached the boundary between two materials of
  different densities, such as air and water, the
  increased gravitational force of the denser
  material would cause the particles to change
  direction, Newton believed (see our Density
  module).
• Newtons particle theory was also based partly on
  his observations of how the wave phenomenon
  diffraction related to sound. He understood that
  sound traveled through the air in waves, meaning
  sound could travel around corners and obstacles,
  thus a person in another room can be heard
  through a doorway. Since light was unable to bend
  around corners or obstacles, Newton believed that
  light could not diffract. He therefore supposed
  light was not a wave.
• Hooke and others most notably the Dutch
  scientist Christian Huygens believed that
  refraction occurred because light waves slowed
  down as they entered a denser medium such as
  water and changed their direction as a result.
  These wave theorists believed, like Descartes,
• that light must travel through some material
  that
• permeates space. Huygens dubbed this medium
  the
• aether.

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• Speed of Light
• The early Greek philosophers generally followed
  Aristotle's belief that the speed of light was
  infinite. 2 As late as 1600 A.D., Johannes
  Kepler, one of the fathers of modern astronomy,
  maintained the majority view that light was
  instantaneous in its travels. Rene Descartes, the
  highly influential scientist, mathematician and
  philosopher (who died in 1650), also strongly
  held to the belief in the instantaneous
  propagation of light. He strongly influenced the
  scientists of that period and those who followed.
  
• In 1677 Olaf Roemer, the Danish astronomer, noted
  that the time elapsed between eclipses of Jupiter
  with its moons became shorter as the Earth moved
  closer to Jupiter and became longer as the Earth
  and Jupiter drew farther apart. This anomalous
  behavior could be accounted for by a finite speed
  of light.
• Initially, Roemer's suggestion was hooted at. It
  took another half century for the notion to be
  accepted. In 1729 the British astronomer James
  Bradley's independent confirmation of Roemer's
  measurements finally ended the opposition to a
  finite value for the speed of light. Roemer's
  work, which had split the scientific community
  for 53 years, was finally vindicated.

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27.2 The Speed of Light

• Over the past 300 years, the velocity of light
  has been measured 163 times by 16 different
  methods. (As a Naval Academy graduate, I must
  point out that Albert Michelson, Class of 1873,
  measured the speed of light at the Academy. In
  1881 he measured it as 299,853 km/sec. In 1907 he
  was the first American to receive the Nobel Prize
  in the sciences. In 1923 he measured it as
  299,798 km/sec. In 1933, at Irvine, CA, as
  299,774 km/sec.)
• The first quantitative estimate of the speed of
  light was made in 1676 by Ole Christensen Rømer,
  who was studying the motions of Jupiter's moon,
  Io, with a telescope. It is possible to time the
  orbital revolution of Io because it enters and
  exits Jupiter's shadow at regular intervals (at C
  or D). Rømer observed that Io revolved around
  Jupiter once every 42.5 hours when Earth was
  closest to Jupiter (at H). He also observed that,
  as Earth and Jupiter moved apart (as from L to
  K), Io's exit from the shadow would begin
  progressively later than predicted. It was clear
  that these exit "signals" took longer to reach
  Earth, as Earth and Jupiter moved further apart.
  This was as a

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• result of the extra time it took for light to
  cross the extra distance between the planets,
  time which had accumulated in the interval
  between one signal and the next. The opposite is
  the case when they are approaching (as from F to
  G). On the basis of his observations, Rømer
  estimated that it would take light 22 minutes to
  cross the diameter of the orbit of the Earth
  (that is, twice the astronomical unit) the
  modern estimate is about 16 minutes and 40
  seconds.
• Around the same time, the astronomical unit was
  estimated to be about 140 million kilometres. The
  astronomical unit and Rømer's time estimate were
  combined by Christiaan Huygens, who estimated the
  speed of light to be 1,000 Earth diameters per
  minute. This is about 220,000 kilometres per
  second (136,000 miles per second), 26 lower than
  the currently accepted value, but still very much
  faster than any physical phenomenon then known.

Rømer's observations of the occultations of Io
from Earth.
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• In 1926, Michelson used a rotating prism to
  measure the time it took light to make a round
  trip from Mount Wilson to Mount San Antonio in
  California, a distance of about 22 miles (36 km).
  The precise measurements yielded a speed of
  186,285 miles per second (299,796 kilometres per
  second).
• Michelsons Method for Measuring the Speed of
  Light
• The diagram below is not to scale.

Light from the source passes through a narrow
slit. It is reflected by face A of the octagonal
metal prism. It then travels a distance, s, (a
few kilometres) and returns to be reflected by
face B. The prism now rotates. If it rotates fast
enough, when light returns to the prism, face B
is no longer in the right position to reflect it
into the observers
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• eye. The image of the slit disappears.
• The speed of rotation is increased. At a certain
  speed of rotation, the image of the slit
  reappears. This is because the time taken for
  light to go from face A to face B was the same as
  the time taken by the prism to rotate 1/8th of a
  revolution.
• If the prism completes n rotations per second
  then the time for one revolution is 1/n.
• Therefore, the time taken for the light to cover
  the distance, s is given by
• So, the speed of light, c is given by
• In 1931, Michelson found c 299774108ms-1.
• The modern value is c 2997925108ms-1

c 8ns
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27.3 Electromagnetic Waves

• Electromagnetic waves exist with an enormous
  range of frequencies. This continuous range of
  frequencies is known as the electromagnetic
  spectrum. The entire range of the spectrum is
  often broken into specific regions. The
  subdividing of the entire spectrum into smaller
  spectra is done mostly on the basis of how each
  region of electromagnetic waves interacts with
  matter. The diagram below depicts the
  electromagnetic spectrum and its various regions.
  The longer wavelength, lower frequency regions
  are located on the far left of the spectrum and
  the shorter wavelength, higher frequency regions
  are on the far right. Two very narrow regions
  within the spectrum are the visible light region
  and the X-ray region. You are undoubtedly
  familiar with some of the other regions of the
  electromagnetic spectrum.
• ROYGBIV
• 
  

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27.4 Light and Transparent Materials

• Transparent  material transmitting light without
  distorting directions of waves.
• Translucent  material transmitting light without
  but distorting its path.
• Opaque  material that does not transmit light.
• These are the three terms that refer to a
  materials ability to transmit light through the
  material and to what degree. We will cover the
  third one in the next section.
• Light passes through materials whose atoms absorb
  the energy and immediately reemit it as light.
• Materials that transmit light are transparent.
  Glass and water are transparent. Visualize the
  electrons in an atom as connected by imaginary
  springs, as shown in Figure 27.6 in your books.
  When light hits the electrons, they vibrate.
• Electrons in glass have a natural vibrational
  frequency in the ultraviolet range. The
  vibration in glass becomes so large that the
  energy is given up in the form of heat, and the
  ultraviolet light is blocked!

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27.5 Opaque Materials

• Materials such as paper, paint, and biological
  tissue are opaque because the light that passes
  through them is scattered in complicated and
  seemingly random ways. A new experiment conducted
  by researchers at the City of Paris Industrial
  Physics and Chemistry Higher Educational
  Institution (ESPCI) has shown that it's possible
  to focus light through opaque materials and
  detect objects hidden behind them, provided you
  know enough about the material. The experiment is
  reported in the current issue of Physical Review
  Letters, and is the subject of Viewpoint in APS
  Physics (physics.aps.org) by Elbert van Putten
  and Allard Moskof the University of Twente.

Knowing enough about the way light is scattered
through materials would allow physicists to see
through opaque substances, such as the sugar cube
on the right. In addition, physicists could use
information characterizing an opaque material to
put it to work as a high quality optical
component, comparable to the glass lens show on
the left. - American Physical Society
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27.6 Shadows

• Shadows
• A shadow is formed where light is 'missing'. A
  dark shadow (umbra) is formed where no light
  falls and a light shadow (penumbra) is formed
  where some light falls, but some is blocked.
• If the light source is very tiny and concentrated
  in one place (a point source) only a sharp shadow
  is formed.

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• If the source is broader light from the top of
  the source causes a lower shadow than that from
  the top. You therefore get partial shadow or
  penumbra as well as umbra.
• If we use coloured lights at different points we
  can see the effect of these multiple shadows

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• The size of a shadow changes as you move the
  source closer or further from the screen
• These terms are used to express ideas in
  astronomy
• So, why are some shadows lighter than others?

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• How dark a shadow is depends on the lighting
  conditions that create it. If there is only once
  point source of light, then when it is blocked,
  no light will reach the shadowed area and the
  shadow will be dark. If there is a lot of
  reflection, diffuse light, or multiple light
  sources, however, the shadow will be lighter.
• Shadows Outside
• On a sunny day, most of the light is coming
  directly from the sun, but some of it is coming
  as blue scattered light coming from the sky. This
  hits you at all angles as it comes from all
  directions. Therefore, if you stand in front of
  the sun, the sun's light is blocked, but your
  shadow still receives light from the rest of the
  sky, and you can still see the shadowed ground.
  On a cloudy day, the light is completely diffuse,
  not coming from anywhere in particular, and you
  don't cast much of a shadow at all.

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27.7 Polarization

• Polarization
• A light wave is an electromagnetic wave which
  travels through the vacuum of outer space. Light
  waves are produced by vibrating electric charges.
  The nature of such electromagnetic waves is
  beyond the scope of The Physics Classroom
  Tutorial. For our purposes, it is sufficient to
  merely say that an electromagnetic wave is a
  transverse wave which has both an electric and a
  magnetic component.
• The transverse nature of an electromagnetic wave
  is quite different from any other type of wave
  which has been discussed in The Physics Classroom
  Tutorial. Let's suppose that we use the customary
  slinky to model the behavior of an
  electromagnetic wave. As an electromagnetic wave
  traveled towards you, then you would observe the
  vibrations of the slinky occurring in more than
  one plane of vibration. This is quite different
  than what you might notice if you were to look
  along a slinky and observe a slinky wave
  traveling towards you. Indeed, the coils of the
  slinky would be

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• vibrating back and forth as the slinky
  approached yet these vibrations would occur in a
  single plane of space. That is, the coils of the
  slinky might vibrate up and down or left and
  right. Yet regardless of their direction of
  vibration, they would be moving along the same
  linear direction as you sighted along the slinky.
  If a slinky wave were an electromagnetic wave,
  then the vibrations of the slinky would occur in
  multiple planes. Unlike a usual slinky wave, the
  electric and magnetic vibrations of an
  electromagnetic wave occur in numerous planes. A
  light wave which is vibrating in more than one
  plane is referred to as unpolarized light. Light
  emitted by the sun, by a lamp in the classroom,
  or by a candle flame is unpolarized light. Such
  light waves are created by electric charges which
  vibrate in a variety of directions, thus creating
  an electromagnetic wave which vibrates in a
  variety of directions. This concept of
  unpolarized light is rather difficult to
  visualize. In general, it is helpful to picture
  unpolarized light as a wave which has an average
  of half its vibrations in a horizontal plane and
  half of its vibrations in a vertical plane.

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• Polarization by Use of a Polaroid Filter
• The most common method of polarization involves
  the use of a Polaroid filter. Polaroid filters
  are made of a special material which is capable
  of blocking one of the two planes of vibration of
  an electromagnetic wave. (Remember, the notion of
  two planes or directions of vibration is merely a
  simplification which helps us to visualize the
  wavelike nature of the electromagnetic wave.) In
  this sense, a Polaroid serves as a device which
  filters out one-half of the vibrations upon
  transmission of the light through the filter.
  When unpolarized light is transmitted through a
  Polaroid filter, it emerges with one-half the
  intensity and with vibrations in a single plane
  it emerges as polarized light.
• Polarization of light by use of a Polaroid filter
  was is often demonstrated in a Physics class
  through a variety of demonstrations. Filters are
  used to look through an view objects. The filter
  does not distort the shape or dimensions of the
  object it merely serves to

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• produce a dimmer image of the object since
  one-half of the light is blocked as it passed
  through the filter. A pair of filters are often
  placed back to back in order to view objects
  looking through two filters. By slowly rotating
  the second filter, an orientation can be found in
  which all the light from an object is blocked and
  the object can no longer be seen when viewed
  through two filters. What happened? In this
  demonstration, the light was polarized upon
  passage through the first filter perhaps only
  vertical vibrations were able to pass through.
  These vertical vibrations were then blocked by
  the second filter since its polarization filter
  is aligned in a horizontal direction. While you
  are unable to see the axes on the filter, you
  will know when the axes are aligned perpendicular
  to each other because with this orientation, all
  light is blocked. So by use of two filters, one
  can completely block all of the light which is
  incident upon the set this will only occur if
  the polarization axes are rotated such that they
  are perpendicular to each other.
•  

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• A picket-fence analogy is often used to explain
  how this dual-filter demonstration works. A
  picket fence can act as a polarizer by
  transforming an unpolarized wave in a rope into a
  wave which vibrates in a single plane. The spaces
  between the pickets of the fence will allow
  vibrations which are parallel to the spacings to
  pass through while blocking any vibrations which
  are perpendicular to the spacings. Obviously, a
  vertical vibration would not have the room to
  make it through a horizontal spacing. If two
  picket fences are oriented such that the pickets
  are both aligned vertically, then vertical
  vibrations will pass through both fences. On the
  other hand, if the pickets of the second fence
  are aligned horizontally, then the vertical
  vibrations which pass through the first fence
  will be blocked by the second fence. This is
  depicted in the
• diagram to the right.

In the same manner, two Polaroid filters oriented
with their polarization axes perpendicular to
each other will block all the light. Now that's a
pretty cool observation which could never be
explained by a particle view of light.
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• Polarization by Reflection
• Unpolarized light can also undergo polarization
  by reflection off of nonmetallic surfaces. The
  extent to which polarization occurs is dependent
  upon the angle at which the light approaches the
  surface and upon the material which the surface
  is made of. Metallic surfaces reflect light with
  a variety of vibrational directions such
  reflected light is unpolarized. However,
  nonmetallic surfaces such as asphalt roadways,
  snow fields and water reflect light such that
  there is a large concentration of vibrations in a
  plane parallel to the reflecting surface. A
  person viewing objects by means of light
  reflected off of nonmetallic surfaces will often
  perceive a glare if the extent of polarization is
  large. Fisherman are familiar with this glare
  since it prevents them from seeing fish which lie
  below the water. Light reflected off a lake is
  partially polarized in a direction parallel to
  the water's surface. Fisherman know that the use
  of glare-reducing sunglasses with the proper
  polarization axis allows for the blocking of this
  partially polarized light. By blocking the
  plane-polarized light, the glare is reduced and
  the fisherman can more easily see fish located
  under the water.

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27.8 Polarized Light and 3-D
Viewing

• Applications of Polarization
• Polarization has a wealth of other applications
  besides their use in glare-reducing sunglasses.
  In industry, Polaroid filters are used to perform
  stress analysis tests on transparent plastics. As
  light passes through a plastic, each color of
  visible light is polarized with its own
  orientation. If such a plastic is placed between
  two polarizing plates, a colorful pattern is
  revealed. As the top plate is turned, the color
  pattern changes as new colors become blocked and
  the formerly blocked colors are transmitted. A
  common Physics demonstration involves placing a
  plastic protractor between two Polaroid plates
  and placing them on top of an overhead projector.
  It is known that structural stress in plastic is
  signified at locations where there is a large
  concentration of colored bands. This location of
  stress is usually the location where structural
  failure will most likely occur. Perhaps you wish
  that a more careful stress analysis was performed
  on the plastic case of the CD which you recently
  purchased.

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• Polarization is also used in the entertainment
  industry to produce and show 3-D movies.
  Three-dimensional movies are actually two movies
  being shown at the same time through two
  projectors. The two movies are filmed from two
  slightly different camera locations. Each
  individual movie is then projected from different
  sides of the audience onto a metal screen. The
  movies are projected through a polarizing filter.
  The polarizing filter used for the projector on
  the left may have its polarization axis aligned
  horizontally while the polarizing filter used for
  the projector on the right would have its
  polarization axis aligned vertically.
  Consequently, there are two slightly different
  movies being projected onto a screen. Each movie
  is cast by light which is polarized with an
  orientation perpendicular to the other movie. The
  audience then wears glasses which have two
  Polaroid filters. Each filter has a different
  polarization axis - one is horizontal and the
  other is vertical. The result of this arrangement
  of projectors and filters is that the left eye
  sees the movie which is projected from the right
  projector while the right eye sees the movie
  which is projected from the left projector. This
  gives the viewer a perception of depth.

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How do 3D movies use polaroid filters?
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A different approach

• Use color filters to make the left and right eyes
  perceiving slightly different images
• http//www.3dmovies.com/