The Nature of Light

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The Nature of Light

• What is light has been a basic philosophical and
  scientific question since time immemorial.

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Isaac Newton

• Known for his Law of Universal Gravitation,
  English physicist Sir Isaac Newton (1643 to 1727)
  realized that light had frequency-like properties
  when he used a prism to split sunlight into its
  component colors, called dispersion.
• This was the first thought that it had wavelike
  characteristics.

3
Isaac Newton

• Nevertheless, he thought that light was a
  particle because the periphery of the shadows it
  created was extremely sharp and clear.

4
Francesco Grimaldi and Christian Huygens

• Light as a wave - The wave theory, which
  maintains that light is a wave, was proposed
  around the same time as Newton's theory. This
  meansthat the explanation for the nature of
  light is that light travels as a transverse wave
  through different mediums. The site below has a
  helpful tutorial about light as a wave model.
  Check out science trek gtunder electromagnetic
  waves.(hi-lite link, right click gtopen hyperlink)
• http//www.colorado.edu/physics/2000/index.pl

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Supporting observations

• In 1665, Italian physicist Francesco Maria
  Grimaldi (1618 to 1663) discovered the phenomenon
  of light diffraction and pointed out that it
  resembles the behavior of waves. Then, in 1678,
  Dutch physicist Christian Huygens (1629 to 1695)
  established the wave theory of light and
  announced the Huygens' principle.

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Observations of diffraction using Light and Shadow
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So, finally

• In 1678, Dutch physicist Christian Huygens (1629
  to 1695) established the wave theory of light and
  announced the Huygens' principle
• when light goes through an aperture (an opening
  within a barrier) every point of the light wave
  within the aperture can be viewed as creating a
  circular wave which propagates outward from the
  aperture

8
Diffraction of light

• Interactive simulation showing diffraction. The
  opening and frequency are adjustible.
• http//micro.magnet.fsu.edu/primer/java/scienceopt
  icsu/diffraction/basicdiffraction/index.html

9
Augustin-Jean Fresnel

• French physicist Augustin-Jean Fresnel (1788 to
  1827) asserted that light waves have an extremely
  short wavelength and mathematically proved light
  interference. In 1815, he devised physical laws
  for light reflection and refraction, as well. He
  also hypothesized that space is filled with a
  medium known as ether because waves need
  something that can transmit them.Ether medium
  ideas didnt hold up.

10
Thomas Young

• In 1817, English physicist Thomas Young (1773 to
  1829) calculated light's wavelength from an
  interference pattern, thereby not only figuring
  out that the wavelength is 1 micron or less, but
  also having a handle on the truth that light is a
  transverse wave.At that point, the particle
  theory of light fell out of favor and was
  replaced by the wave theory.

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In Youngs Experiment

• The waves from the two sources are in phase at
  the center. Bright areas are caused by
  constructive interference and the dark areas are
  caused by destructive interference.
• As the distance from the center increases, the
  path traveled by the light from one source is
  larger than that traveled by the light from the
  other source.

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Youngs experiment continued

• When the difference in path is equal to half a
  wavelength, destructive interference occurs.
  Instead, when the difference in path length is
  equal to a wavelength, constructive interference
  occurs.
• For destructive interference the waves are 180
  degrees out of phase and for constructive
  interference they are exactly in phase.

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Youngs Double Slot Diffraction Experiment

• Measurements from this type of experiment was
  used to determine the wavelength of light.
• http//vsg.quasihome.com/interfer.htm

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Maxwell

• Maxwell's four equations have become the most
  fundamental law in electromagnetics. The
  equations actually predicted the existence of
  electromagnetic waves in 1861before we had
  devised technologies to detect details of
  electromagnetic radiation.

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Light as a particleA. Einstein

• The theory of light being a particle completely
  vanished until the end of the 19th century when
  Albert Einstein revived it.Now that the dual
  nature of light as "both a particle and a wave"
  has been proved, its essential theory was further
  evolved from electromagnetics into quantum
  mechanics..

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Model Depicting the Duality of Light

• Model Depicting the Duality of Light

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Einsteins Quantum Theory

• Einstein believed light is a particle (photon)
  and the flow of photons is a wave.
• The main point of Einstein's light quantum theory
  is that light's energy is related to its
  oscillation frequency.
• Photon energy is the height of the oscillation
  frequency and the intensity of light is the
  quantity of photons.
• This site presents the photon theory for light is
  a particle.
• http//www.colorado.edu/physics/2000/index.pl
  click on
• gtscience trek, gtquantum atom, scroll to bottom,
  gtnext, gt click on energy levels and watch photons.

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Einsteins Photoelectric Effect

• Albert Einstein (1879 to 1955), famous for his
  theories of relativity, conducted research on the
  photoelectric effect, in which electrons fly out
  of a metal surface exposed to light.The strange
  thing about the photoelectric effect is the
  energy of the electrons (photoelectrons) that fly
  out of the metal does not change whether the
  light is weak or strong. (If light were a wave,
  strong light should cause photoelectrons to fly
  out with great power.)Another puzzling matter
  is how photoelectrons multiply when strong light
  is applied. Einstein explained the photoelectric
  effect by saying that "light itself is a
  particle," and for this he received the Nobel
  Prize in Physics.
• http//www.canon.com/technology/s_labo/light/001/1
  1/014.html

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Einsteins Oscillation Frequency

• The main point of his light quantum theory is the
  idea that light's energy is related to its
  oscillation frequency). In short, Einstein was
  saying that light is a flow of photons, the
  energy of these photons is the frequency of their
  oscillation frequency, and the intensity of the
  light is the quantity of its photons.
• http//www.canon.com/technology/s_labo/light/001/1
  1/015.html

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Photoelectric EffectClick on the icon below for
an interactive simulation.
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Einsteins Quantum Theory

• Einstein proved his theory by proving that the
  Planck's constant which he developed, exactly
  matched the Plancks constant of 6.6260755 x
  10-34 that Planck independently discovered from
  his experiments.

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Einsteins Quantum Theory

• This showed light as a wave and the properties of
  light as both a particle and a wave.
• http//www.canon.com/technology/s_labo/light/001/1
  1/016.html

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Electromagnetic Spectrum

• All electromagnetic energy is a continuum,
  gradually changing in wavelength from less than a
  billionth of a meter to many miles long. All
  forms of energy within this group travel as
  electrical energy in one plane and magnetic
  energy oriented 90 degrees and in phase with the
  electric field.

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Electromagnetic Spectrum
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Electromagnetic Spectrum

• Makes up all of the radiation in the universe,
  some visible and most invisible.
• All forms of radiation within the spectrum ,
  186,000 miles per second, travel at the speed of
  light in a vacuum.
• Radiation travels through some mediums and
  without a medium as well.

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Light Intensity

• Light Intensity is inversely proportional to the
  square of the distance I 1/d2 .
• According to Einstein, the intensity of the light
  is determined by the quantity of the photons
  hitting the surface.

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Electromagnetic spectrum continued

• We see only a very narrow portion of the
  radiation of the spectrum, the rest simply lies
  outside the threshold of our eyes to detect it
  and is therefore invisible to our eyes.

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Parts of the electromagnetic spectrum

• Gamma Radiation invisible, shortest wavelength,
  highest frequency, highest energy radiation, can
  penetrate up to 3 m of concrete. Source
  radioactive elements. Used to treat cancer.
• X-ray radiation invisible, next highest energy,
  passes through skin and soft tissue, but absorbed
  by bone.

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Electromagnetic spectrum continued

• Ultra-violet radiation, penetrates skin cells,
  causes sunburn, cellular mutations (DNA damage)
  and skin cancer (melanoma)
• Visible Light The only part of the EMS you can
  see, broken down to different wavelengths (400
  nm violet 700 nm red) and frequencies and
  perceived as different color ROYGBIV

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Electromagnetic Spectrum continued

• Infrared Light- or heat radiation - although we
  can not see it directly, we can detect it with
  heat sensors embedded in our skin.
• Its wavelength is longer than red light
• Microwave radiation longer wavelength than IR,
  it is used in microwaves to heat water in food
  and for cellular communication.

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Electromagnetic spectrum final

• Radio waves composed of waves lengths which are
  assigned to AM radio, FM radio, TV, and radar.
  They have the longest wavelength and the lowest
  frequency and the lowest energy.

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Why does this happen? Lets see..
Same candles viewed in red light
Candles viewed in white light
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Color Filters

• Filters are placed between the object and the
  observer. They work by letting a specific
  frequency pass through ONLY and the rest of the
  frequencies are blocked or absorbed.
• You see the object only because of the frequency
  that reaches your eye, the color you perceive is
  assembled by your brain as it receives signals
  from the stimulation of the cone receptors inside
  your eye.

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Color Filter Interactive Demo Site

• http//micro.magnet.fsu.edu/primer/java/primarycol
  ors/colorfilters/index.html

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Additive Color Light
Light is perceived as white by humans when all
three cone cell types of the eye are
simultaneously stimulated by equal amounts of
red, green, and blue light. These are called
Primary colors, because when they are added
together, white light is formed.
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Primary Additive Color Interactive Tutorial Site

• Primary Additive Color Interactive Tutorial Site

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Subtractive Color

• The complementary colors (cyan, yellow, and
  magenta) are also commonly referred to as the
  primary subtractive colors because each can be
  formed by subtracting one of the primary
  additives (red, green, and blue) from white
  light.
• The color observed by subtracting a primary color
  from white light results because the brain adds
  together the colors that are left to produce the
  respective complementary or subtractive color.

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Check this out!
Subtractive Color Interactive Site

• Subtractive Color Interactive Site

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Color perception and color separation.

• Pigments and dyes within objects are responsible
  for most of the color that we perceive.
• When any two of the wave frequencies of the
  primary subtractive colors are added, they
  produce a primary additive color. For example,
  adding magenta and cyan together produces the
  color blue, while adding yellow and magenta
  together produces red and adding yellow and cyan
  is interpreted as green.

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Color separation continued

• When all three primary subtractive colors are
  added, the three primary additive colors are
  removed from white light leaving black (the
  absence of any color).
• White cannot be produced by any combination of
  the primary subtractive colors, which is the main
  reason that no mixture of colored paints or inks
  can be used to print white.

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Color Separation Interactive tutorial

• http//micro.magnet.fsu.edu/primer/java/primarycol
  ors/colorseparation/index.html

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Paint theory

• Paint is produced from Base pigments which
  contain the subtractive primaries that are mixed
  together. Depending what pigments that are mixed
  together initially, a specific color combination
  is created in final paint. The final color that
  is perceived is determined by the amount of
  signals the brain receives from the different
  frequencies in the light that strikes the eye and
  are detected by the cones and sent to the brain
  where the combination of signals are processed.

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White light illuminates objects A and B
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Pigments
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Pigments in various light
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Reflection

• Occurs when radiation transitions from one
  density to another different density.
• http//www.physicsclassroom.com/mmedia/waves/ltm.g
  if

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Reflection from Plane Mirror

• Reflection is the bouncing off of radiation, or
  visible light, from a surface.

The angle of incidence is equal to the angle of
reflection.  
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Virtual Image for Plane Mirror
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Plane Mirror, virtual image and ray path

• http//www.4physics.com/phy_demo/flat_mirror/mirro
  r.html

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Concave Mirror ray paths

• http//www.4physics.com/phy_demo/mirage/focal-leng
  th.html

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Refraction

• The bending of light ray caused by transitioning
  from one medium to another of medium of a
  different density.

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Refraction ray path

• http//www.ps.missouri.edu/rickspage/refract/refra
  ction.html scroll down to the refraction
  simulator, Figure 3. It is interactive.

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Convex Lens Refraction
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Convex Lens Refraction Rules

• Any incident ray traveling parallel to the
  principal axis of a converging lens will refract
  through the lens and travel through the focal
  point on the opposite side of the lens.
• Any incident ray traveling through the focal
  point on the way to the lens will refract through
  the lens and travel parallel to the principal
  axis.
• An incident ray which passes through the center
  of the lens will in effect continue in the same
  direction that it had when it entered the lens.

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Bi-Convex lens Real and Inverted Image
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Bi-Convex Virtual and Upright Image
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Bi-Convex Image location and Size
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Bi-Concave Lens Refraction
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Refraction Rules for Divergent (Bi-concave) Lens

• Any incident ray traveling parallel to the
  principal axis of a diverging lens will refract
  through the lens and travel in line with the
  focal point (i.e., in a direction such that its
  extension will pass through the focal point).
• Any incident ray traveling towards the focal
  point on the way to the lens will refract through
  the lens and travel parallel to the principal
  axis.
• An incident ray which passes through the center
  of the lens will in effect continue in the same
  direction that it had when it entered the lens.

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Rainbows
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Rainbow Refraction ray path