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Light and Color

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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? ... – PowerPoint PPT presentation

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Title: Light and Color


1
Light and Color
  • Chapter 27 and 28

2
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.

3
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
4
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.

5
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
6
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
7
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
8
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
9
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.

10
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?

11
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
12
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

13
Primary Colors of Light
White
Red
Primary Colors
Green
Blue
Yellow
Cyan
Magenta
14
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
15
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
16
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.

17
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18
Color Blindness
  • Colorblindness - about 10 of population
  • Red-green is predominant
  • Yellow-blue - a few
  • Total - some

19
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20
Optical Illusions
21
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22
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23
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
24
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
25
Things that can separate white light
  • Prism
  • Raindrops
  • CDs
  • Diffraction Grating

26
Things the produce electromagnetic waves
  • Radio waves
  • electrons moving up and down an antenna
  • Visible Light
  • electrons changing energy states in an atom

27
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.

28
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.

29
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
30
Every element can be fingerprinted by it
spectra.
Hydrogen
Helium
Oxygen
Carbon
31
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
32
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
33
Absorption Spectra of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light Source
Prism
Photographic Film
34
Sources
  • Conceptual Physics by Paul Hewitt
  • www.physicsclassroom.com
  • http//observe.phy.sfasu.edu/courses/phy101/lectur
    es101/
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