Properties

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Chapter 27

• Properties
• of
• Light

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Models of Light
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Models of Light

1. Newton introduced the particle theory. He said
   that light travels in straight lines unlike
   waves.

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Models of Light

1. Huygen disagreed with Newton and said that light
   is a wave because it spreads out sometimes.

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Models of Light

1. Einstein realized that light acts not only as
   waves but as quantum particles now known as
   photons.
2. Light has dual properties Particles and Waves

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• Question Does light have mass?

Black Holes

• Once was a star at least twice the mass of the
  sun that exhausted its fuel and collapsed to
  infinite density

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A ravenous black hole This is a Hubble Space
Telescope image of an 800-light-year-wide
spiral-shaped disk of dust fueling a massive
black hole in the center of galaxy NGC 4261,
located 100 million light-years away in the
direction of the constellation Virgo. By
measuring the speed of gas swirling around the
black hole, astronomers calculate that the object
at the center of the disk is 1.2 billion times
the mass of our sun.
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Speed of Light

• A. Roemer was the first person to try to measure
  the speed of light in observations of Io.

• His calculation was 300,000 km/s.

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Speed of Light

• In 1880, Albert Michelson measured the speed of
  light to be 299,920 km/s.
• Used spinning octagonal mirrors and a mirror far
  away
• Adjusted spinning speed to calculate speed of
  light

• In 1907, he won the Nobel prize in physics.

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Electromagnetic Wave Velocity

• Light could go around the world 7.5 times each
  second

http//liftoff.msfc.nasa.gov/academy/rocket_sci/sa
tellites/geo-high.html
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Electromagnetic Wave Velocity

• Light takes 8 minutes to travel from the sun to
  earth.

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Electromagnetic Wave Velocity

• The next closest star is 4 light years away
• The diameter of our galaxy is 100,000 light
  years
• Some galaxies are 10 billion light years away

http//bluepoint.egenet.net/sagan/galaxy.gif
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Electromagnetic Spectrum

• Energy moving at speed of light
• Vary by f and l only
• Light is only a small part
• Below light is infrared, above is ultraviolet

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Electromagnetic Spectrum

• Radio Waves communication TV and Radio
• Microwaves cooking cell phones
• Infrared heat waves
• Visible Light detected by your eyes
• Ultraviolet causes sunburns
• X-rays penetrates tissue
• Gamma Rays most energetic

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• The Visible Spectrum
• A range of light waves extending in wavelength
  from about 400 to 700 nanometers
• Less than 1 of the entire spectrum

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Questions

• Is it correct to say that radio wave is a
  low-frequency light wave?
• Is a radio wave also a sound wave?

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Transparent Materials

• Transparent - the term applied to materials
  through which light can pass in straight lines

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Visible Light and Glass

• Visible light maintains the same frequency when
  it enters glass
• But the velocity changes
• Light travels 0.75x the original speed in water
• In glass 0.67x.
• In diamond 0.41x
• When the light comes out it regains its speed

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Opaque Materials

• Opaque - the term applied to materials that
  absorb EM

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Opaque Materials

• Opaque materials absorb EM
• ex.) wood, metal, rocks ,etc.
• The light energy is transformed into random
  kinetic energy
• makes the object warmer because it absorbs the
  electromagnetic energy

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http//www.ch.ic.ac.uk/local/projects/steer/chloro
.htm
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Example Questions

• Are clouds transparent or opaque to visible
  light?
• Answer opaque
• Are clouds transparent or opaque to ultraviolet
  light?
• Answer transparent

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Example Questions

• Are windows transparent or opaque to visible
  light?
• Answer transparent
• Are windows transparent or opaque to ultraviolet
  light?
• Answer opaque
• Due to Law of Conservation of Energy the EM is
  given off as heat

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Shadows

• Umbra - the darker part of a shadow where all the
  light is blocked
• Penumbra - a partial shadow

These terms also apply to Solar Eclipses and
Lunar Eclipses.
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Solar Eclipse

• A solar eclipse occurs when the Moon passes in
  front of the Sun.

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A lunar eclipse occurs when the Moon passes into
the Earth's shadow.
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Questions

• Which type of eclipse is dangerous to view with
  the unprotected eye?
• Why are lunar eclipses more commonly seen than
  solar eclipses?

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Seeing the Light The Eye

• Cornea -

does most of the focusing

• Iris -

has the eye color and controls light intensity

• Pupil -

the hole in the eye

• Lens -

does remainder of focusing

• Retina -

location of light sensors, has rods and cones
center of vision, predominantly cones

• Fovea -

• Blind spot -

optic nerve exit, no light sensors
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Parts of the Eye

• Detectors on the Fovea
• Rods
• light intensity and motion sensitive
• Cones
• color sensitive
• The blind spot for the eye is cause by the optic
  nerve.

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p. 474, Conceptual Physics 2002
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Color Vision!

• Colorblindness - about 10 of the population
• Red-green is predominant
• Yellow-blue - a few
• Total some
• Mostly males X-linked trait

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Color Deficiency
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Optical Illusions
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Optical Illusions
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Optical Illusions
Are the horizontal lines parallel, or do they
slope?
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Optical Illusions
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Optical Illusions
During the Optical Art (OpArt) Movement of the
1960s, artists would create all sort of puzzling
effects with color. This "flashing squares"
drawing seems to wobble and flash when you
concentrate on one particular area of the image.
How many squares can you see in this diagram?
Can you feel the "motion" of the image?
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Optical Illusions
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Optical Illusions
Can you count the black dots?
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Optical Illusions
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Myopia (Near-Sightedness)
People with near-sightedness cannot see clearly
at distance.
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Hyperopia (Farsightedness)
People with far-sightedness cannot see clearly up
close
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You will observe a total eclipse of the sun
when...
(a) youre in the penumbra of the moons
shadow (b) youre in the umbra of the moons
shadow (c) sunlight diffracts around the
moon (d) sunlight reflects from the moon to the
earth
(b) youre in the umbra of the moons shadow
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The speed of light...

• (a) has never been measured
• (b) is about the same as that of sound
• (c) is infinitely fast
• (d) is very fast, but not infinite

(d) is very fast, but not infinite
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In the dark in late evening no color is seen
because of lack of stimulation of

• a. rods.
• b. cones.
• c. cornea.
• d. crystalline lens.

b. cones.
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Self TestOn a Blank Page....

• List the Seven Forms of Light
• Which has the longest wavelength?
• Which has the highest frequency
• Which has the highest energy?
• Which has the fastest speed?

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Structure of the Atom
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Ground State of Electron

• Energy level it normally occupies
• state of lowest energy for that electron

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Excited State of Electron

• Temporarily energy state greater than its ground
  state
• e- can become excited if it is given extra energy
• absorbs a photon, or packet of light
• or collides with a nearby atom or particle

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Emission of Photon

• Electrons do not stay in excited states for very
  long
• they soon return to their ground states
• emitting a photon with the same energy as the
  one that was absorbed

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Transitions among the various orbitals are unique
for each element because the energy levels are
uniquely determined by the protons and neutrons
in the nucleus
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• When the electrons of a certain atom return to
  lower orbitals from excited states, the photons
  they emit have energies that are characteristic
  of that kind of atom
• This gives each element a unique fingerprint,
  making it possible to identify the elements
  present in a container of gas, or even a star

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Kirchhoff-Bunsen Experiment
These two scientists found that burning chemicals
over an open flame resulted in an emission
spectrum with bright lines

• They found that each chemical element produced
  its own characteristic pattern of bright spectral
  lines

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Every element can be fingerprinted by it
spectra.
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Incandescence

• Hot, dense solids produce a continuous spectrum.
  

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The color of light emitted by a hot object
changes with its temperature
hottest glowing object ? Bluish
White

Yellowish

Orange-ish

coolest glowing object ? Reddish
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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.

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Matching Questions

• Type of Spectrum Appearance
• 1. Emission Spectra a. All Colors
• 2. Continuous Spectra b. Dark Lines
• 3. Absorption Spectra c. Bright Lines

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Matching Questions

• 1. Emission Spectra a. Hot Solids
• 2. Continuous Spectra b. Hot Stars
• 3. Absorption Spectra c. Hot Gases

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Fluorescence

• Some materials that are excited by ultraviolet
  light can emit visible light upon de-excitation
• The materials electrons use some of the energy
  during vibration
• The emitted light (bluish white) is at a lower
  frequency (energy) than the UV

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Fluorescent Lamps

• Primary excitation - electron collisions with low
  pressure mercury vapor, and ultraviolet light is
  given off
• Secondary excitation - ultraviolet light is
  absorbed by phosphors and these emit visible
  light

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Phosphorescence

• Phosphorescence - a type of light emission that
  is the same as fluorescence except for a delay
  between excitation and de-excitation.
• Electrons get "stuck" in an excited state and
  de-excite gradually.

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Lasers

• Light Amplification by Stimulated Emission of
  Radiation
• Lasers produce coherent light.
• all the light waves have the frequency, phase and
  direction.
• Demo - Laser and chalk dust

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Matching Questions

• Incandescence a. mercury vapor
• light tubes
• 2. Fluorescence b. glow-in-the-dark
• paints and plastics
• 3. Phosphorescence c. light bulbs
• with filaments

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Matching Questions

• 1. Fluorescent Lamp a. color changes
• with temperature
• 2. Incandescent Bulb b. only one color
• of light
• 3. Laser c. converts ultraviolet
  light to visible light

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Chapter 27
Color
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Color

• Different wavelengths of light are perceived as
  different colors
• Pure Colors ROY G. BIV
• White light contains equal amounts of these
  colors (ROYGBIV)

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Selective Reflection and Transmission

• At an interface, light can be...
• absorbed
• reflected
• transmitted

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Fill in the Blanks
absorb

• Black objects _______ all colors of light.
• White objects _______ all colors of light.
• Transparent objects _______ all colors of light

reflect
transmit
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Color Filters

• Red filters transmit red light and absorb the
  other colors, etc.

Red objects reflect red light and absorb the
rest, etc.
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Mixing Colored Light Color Addition

• Additive Primary Colors of Light
• Red
• Green
• Blue
• One can produce any color by varying amplitude
  and mixture or red, green, and blue light.

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Color Addition Example

• Tiny dots called pixels on Color TV's and
  Computer Monitors are colored only red, green, or
  blue

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Mixing Colored Pigments - Color Subtraction

• Subtractive Primary Colors
• Yellow
• Magenta
• Cyan
• One can produce any color by varying the amount
  of yellow, magenta and cyan pigments.

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Green Pigment
Red Pigment
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Colors of Light
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Yellow Pigment
Cyan Pigment

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Yellow Cyan Green Pigment


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In-Class Exercise

1. Make a list of the seven forms of light in order
   of decreasing wavelength.
2. Draw a Color Addition Diagram using overlapping
   colored spotlights. Label all 7 colors.
3. Draw a Color Subtraction Diagram using
   overlapping paints. Label all 7 colors.

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Colors of Light
magenta
red
blue
cyan
yellow
white
green
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yellow
green
cyan
blue
black
red
magenta
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Why is the Sky Blue?

• Nitrogen and Oxygen
• in our atmosphere
• scatter high frequencies of light

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Why is the Ocean Greenish Blue?

• Red light is absorbed by the molecules in the
  water

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Why are Sunsets Red?

• Red light is scattered the least by our
  atmosphere
• The greatest path of sunlight through the
  atmosphere is at sunset or sunrise

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Why are Clouds White?

• Clouds are composed of water droplets
• Different-sized droplets create different types
  of scattered frequencies
• Small droplets scatter blue
• Bigger scatter higher frequencies (like green)
• Even bigger droplets scatter red
• This creates a cloud that appears white

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• Complementary Light Colors - any two colors that
  add together to produce white
• e.g. magenta green white

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After Images - Conal Fatigue

• The human eye will see complimentary colors after
  staring at a color picture.
• Demos
• Colored Shapes
• Texas Flag
• American Flag
• Rose
• Lincoln

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• If you look at the yellow Sun just before sunset
  for a few seconds and then look at a white cloud
  you see _______ afterimages of the Sun.
• a) red
• b) green
• c) blue
• d) cyan

?
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• A mixture of cyan and yellow paints gives
  __________ paint.
• a) green
• b) red
• c) black
• d) white
• e) blue

?
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• Adding red and green light gives us what color?
• a) yellow
• b) cyan
• c) blue
• d) magenta

?
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• Suppose that two flashlight beams are shone on a
  white screen, one through a pane of blue glass
  and the other through a pane of green glass. What
  color appears on the screen where the two beams
  overlap?
• a) yellow
• b) green
• c) cyan
• d) magenta
• e) red

?
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• If a car headlight emitted only yellow light, the
  normally green grass appears to be
• a) green.
• b) red.
• c) black.
• d) white.
• e) yellow.

?
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• To see an after image of a red, white and blue
  Texas flag one would first stare at a flag with
  the complimentary colors
• a) yellow, violet, and green.
• b) cyan, yellow, and magenta.
• c) cyan, black, and yellow.
• d) yellow, cyan and green.

?
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• The worst thing that you can do for the health of
  a green-leafed plant is to illuminate it with
  only
• a) red light.
• b) green light.
• c) blue light.
• d) all are equally bad.
• e) none of these.

?
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Most of the light that we see has undergone
(a) selective interference (b) selective
transmission (c) selective reflection (d) selectiv
e refraction
(c) selective reflection
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A mixture of magenta and green lights give white
light. These two colors are

• (a) additive primaries
• (b) secondary colors
• (c) complementary colors
• (d) fluorescent colors
• (e) interference colors

(c) complementary colors
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Mixing yellow paint and magenta paint gives what
color?
?

• (a) red
• (b) green
• (c) blue
• (d) cyan

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What color would red cloth appear if it were
illuminated by cyan light?

• (a) cyan
• (b) red
• (c) yellow
• (d) green
• (e) black

?