Light and Color

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Light and Color

• Chapter 27 and 28

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Light Wave or particle?

• For many years scientists argued over the nature
  of light, "Is light a wave or a stream of
  particles?"
• In some experiments light exhibits wave like
  properties, the Doppler effect, interference,
  refraction, diffraction
• and in other experiments, like the photo
  electric effect, it exhibits particle like
  properties
• The fact is that light exhibits behaviors which
  are characteristic of both waves and particles.

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Models of Light - Waves

• Electromagnetic waves (light) originate from
  vibrating or accelerating electric charges
• Electromagnetic waves are made up of an electric
  field and a magnetic field oscillating at right
  angles relative to one another

• An electromagnetic wave (light) is a transverse
  wave

Unlike other waves, light waves can travel
through a vacuum
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Models of Light - Particles

• Particle of light are called photons
• Photons have zero rest mass and travel at the
  speed of light through a vacuum.

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Light and Energy

• For waves, the amplitude or intensity is usually
  related to the energy of the wave
• For light, this is not true. The energy of light
  waves was found to be directly related it is
  frequency.
• An experiment demonstrating the photoelectric
  effect demonstrated the particle nature of light
  and that E hf, where E is energy, h is Planks
  constant, and f is frequency.

http//phet.colorado.edu/new/simulations/sims.php?
simPhotoelectric_Effect
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The Electromagnetic Spectrum
The electromagnetic spectrum is the range of
electromagnetic waves extending from radio waves
to gamma rays
Increasing frequency
R O Y G B I V
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The Visible Spectrum

• We can only see a small part of the
  electromagnetic spectrum
• The visible spectrum is a range of light waves
  extending in wavelength from about 400 to 700
  nanometers.

Increasing wavelength
Increasing frequency
Increasing energy
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Speed of Light - c

• In the early 17th century, many scientists
  believed that there was no such thing as the
  "speed of light" they thought light could travel
  any distance in no time at all.
• In the 1670's Roemer was able to calculate a
  value for the speed of light by carefully
  studying the orbit of one of Jupiters moons, Io.
  He noticed that the time between the eclipses of
  the moons of Jupiter was less as the distance
  away from Earth is decreasing than when it is
  increasing.
• In 1926 scientist Albert Michelson used the
  reflection from a rotating mirror on a distant
  mountain and measured the speed of light at
  299,796 km/second
• The current accepted value is 300,000,000 meters
  per second (3 x 108 m/s) or 186,000 miles per
  second.

Light waves obey the wave equation, c lf
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Transparent vs. Opaque

• Transparent - the term applied to materials
  through which light can pass in straight lines.
  If the object is transparent, then the vibrations
  of the electrons are passed on to neighboring
  atoms through the bulk of the material and
  reemitted on the opposite side of the object.
• Opaque - the term applied to materials that
  absorb light. If the object is opaque, then the
  vibrations of the electrons are not passed from
  atom to atom through the bulk of the material
  rather the electrons vibrate for short periods of
  time and then reemit the energy as a reflected
  light wave.

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Light Questions

• Rank the following in terms of energy, frequency
  and wavelength microwaves, visible light, IR,
  UV, x-rays, gamma rays, and radio waves.
• Rank the following in terms of energy, frequency
  and wavelength Red, orange, yellow, blue, and
  green.
• Why is it possible to get a sunburn with the
  window down in the car, but not when the window
  is up?
• Find the frequency of 80 m radio waves.
• What did the photoelectric effect show about
  light?
• Is light a wave or a particle?
• What is the name of a particle of light?
• What is the origin of light?
• Why is light also called electromagnetic
  radiation?
• Why is it possible to see a cell phone ringing in
  a bell jar (vacuum jar) but not possible to hear
  it ringing?
• Do radio waves travel at the speed of light or
  the speed of sound?

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Color

• The color of an opaque object is the result of
  the color of the light which strikes the object
  and the color of the light that it reflects.
• The color of a transparent object is the result
  of the color of the light which strikes the
  object and the color of the light that it
  transmits.
• Red filters transmit red light and absorb the
  other colors, etc.
• Red objects reflect red light and absorb the
  rest, etc.

green
red
white (
)
green
red
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Visible Colors

• Different wavelengths of light are perceived as
  different colors.
• Pure Colors ROYGBIV
• White light contains equal amounts of these
  colors. (ROYGBIV)
• At an interface, light can be...
• absorbed
• reflected
• transmitted

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Primary Colors of Light
White
Red
Primary Colors
Green
Blue
Yellow
Cyan
Magenta
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Combining Colors of Light

• When two of primary colors of light are combined
  they form the primary colors of pigment yellow,
  magenta and cyan
• Combining colors of light is an additive process,
  when all three primary colors are combined, you
  get white light
• Color Addition Example
• Tiny dots called pixels on Color TV's and
  Computer Monitors are colored only red, green, or
  blue.

Red Green Yellow
Green Blue Cyan
Red Blue Magenta
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Complementary Colors

• Complementary Colors - any two colors that add
  together to produce white

Complementary Colors of Light Red and Cyan Green
and Magenta Blue and Yellow
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Primary Colors of Pigment (Subtractive Colors)

• Subtractive Primary Colors
• Yellow
• Magenta
• Cyan
• One can produce any color by varying the amount
  of yellow, magenta and cyan pigments
• Combining pigments is a subtractive process
  because each color absorbs a color or colors of
  light. Combining all three primary colors
  produces black (all colors are absorbed)
• Color Subtraction Example
• Newspapers and zip-lock sandwich bags (yellow and
  cyan makes green) use color subtraction.

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Color Blindness

• Colorblindness - about 10 of population
• Red-green is predominant
• Yellow-blue - a few
• Total - some

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Optical Illusions
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Color Questions

• List the primary colors of light. What happens
  when combine the three primary colors of light?
• List the primary colors of pigment. What happens
  when combine the three primary colors of pigment?
• _______ green yellow Blue _______
  cyan Yellow _______ white Cyan _______
  white
• You have a blue opaque object (an object that
  appears blue under white light). What color will
  it look under the following colors of light?
• Blue
• Red
• Green
• Cyan
• Yellow
• What color does a red shirt appear when the room
  lights are turned off and the room is entirely
  dark? ____________ What about a blue shirt?
  ____________ ... a green shirt? ____________

White
blue
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More Color Questions

• 6) Fill in the blanks
• 7) True or False/Explain White and black are
  colors of light.
• 8) You have a cyan opaque object (an object that
  appears cyan under white light). What color will
  it look under the following colors of light?
• Blue
• Red
• Green
• Cyan
• Yellow

cyan
White
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Things that can separate white light

• Prism
• Raindrops
• CDs
• Diffraction Grating

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Things the produce electromagnetic waves

• Radio waves
• electrons moving up and down an antenna
• Visible Light
• electrons changing energy states in an atom

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The Structure of the Atom and Emission

• An atom is composed of electrons, protons and
  neutrons.
• When an electron is raised to a higher energy
  level, the atom is said to be excited.
• When the electron returns to a lower energy
  level, energy is released in the form of light.
• Different transitions from high levels to low
  levels result in different colors of light.

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The Kirchhoff-Bunsen Experiment

• These two scientists found that burning chemicals
  over an open flame resulted in a spectrum with
  bright lines.
• They found that each chemical element produced
  its own characteristic pattern of bright spectral
  lines.

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Emission Spectra of Hydrogen
Hot gas produces a bright line emission spectrum.

Discrete Emission Spectrum
Slit
Film
Low Density Glowing Hydrogen Gas
Prism
Photographic Film
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Every element can be fingerprinted by it
spectra.
Hydrogen
Helium
Oxygen
Carbon
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Incandescence

• Hot, dense solids produce a continuous spectrum.
• The brightness and color of light emitted by a
  hot object changes with its temperature.
• Glowing object colors
• Reddish ? coolest glowing object
• Orange-ish
• Yellowish
• White
• Bluish ? hottest glowing object

Continuous Spectrum
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Absorption Spectra

• Cool gas in front of a continuous source of light
  produces an absorption line spectrum.
• Fraunhofer lines in our Sun's spectrum showed
  that cool helium gas surrounds the Sun

Absorption Spectrum
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Absorption Spectra of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light Source
Prism
Photographic Film
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Sources

• Conceptual Physics by Paul Hewitt
• www.physicsclassroom.com
• http//observe.phy.sfasu.edu/courses/phy101/lectur
  es101/