1. Properties of
Light
Chapter 27

2. Models of Light
Socrates and Plato believed "streamers" or antennas were emitted from the eyes.
Euclid and Descartes had similar theories.
Greeks believed anything that could be seen emitted light waves.

3. Models of Light
Newton introduced the particle theory. He said that light travels in straight lines unlike waves.

4. Models of Light
Huygen disagreed with Newton and said that light is a wave because it spreads out sometimes.

5. Models of Light
Einstein realized that light acts not only as waves but as quantum particles now known as photons.
Light has dual properties – Particles and Waves

9. Black Holes Question: Does light have mass?
An event horizon exists
inside which the intense gravitational field of the black hole prevents any type of electromagnetic radiation including light from escaping
Once was a star at least twice the mass of the sun that exhausted its fuel and collapsed to infinite density

11. 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.

13. 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.

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

15. Electromagnetic Wave Velocity
Light could go around the world 7.5 times each second

16. Electromagnetic Wave Velocity
Light takes 8 minutes to travel from the sun to earth.

17. 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

19. 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

20. 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

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

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

24. Transparent Materials
Transparent - the term applied to materials through which light can pass in straight lines

25. 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

26. Opaque - the term applied to materials that absorb EM
Opaque Materials
Opaque - the term applied to materials that absorb EM

27. 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

29. Example Questions Are clouds transparent or opaque to visible light?
Answer: opaque
Are clouds transparent or opaque to ultraviolet light?
Answer: transparent

30. 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

31. 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.

32. Solar Eclipse
Umbra
Penumbra
A solar eclipse occurs when the Moon passes in front of the Sun.

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

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

37. 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
Fovea -
center of vision, predominantly cones
Blind spot -
optic nerve exit, no light sensors

39. 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.

40. p. 474, Conceptual Physics 2002

42. Color Vision! Colorblindness - about 10% of the population
Red-green is predominant
Yellow-blue - a few
Total – some
Mostly males – X-linked trait

43. Color Deficiency

44. Optical Illusions

45. Optical Illusions

46. Are the horizontal lines parallel, or do they slope?
Optical Illusions
Are the horizontal lines parallel, or do they slope?

47. Optical Illusions

48. 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?

49. Optical Illusions

50. Can you count the black dots?
Optical Illusions
Can you count the black dots?

51. Optical Illusions

52. Myopia (Near-Sightedness)
People with near-sightedness cannot see clearly at distance.

53. Hyperopia (Farsightedness)
People with far-sightedness cannot see clearly up close

57. You will observe a total eclipse of the sun when...
(a) you’re in the penumbra of the moon’s shadow
(b) you’re in the umbra of the moon’s shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the earth
(b) you’re in the umbra of the moon’s shadow

58. 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

59. 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.

60. Self Test On 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?

62. Structure of the Atom
Neutron
Proton
Electron
Energy Levels
or Orbits

64. Ground State of Electron
Energy level it normally occupies
state of lowest energy for that electron

65. 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

67. 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

69. 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

70. 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

72. Kirchhoff-Bunsen Experiment
These two scientists found that burning chemicals over an open flame resulted in an emission spectrum with bright lines
Emission Spectrum
They found that each chemical element produced its own characteristic pattern of bright spectral lines

73. Hydrogen
Helium
Oxygen
Carbon
Every element can be “fingerprinted” by it spectra.

74. Hot, dense solids produce a continuous spectrum.
Incandescence
Hot, dense solids produce a continuous spectrum.
Continuous Spectrum

75. 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

76. 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

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

79. Matching Questions 1. Emission Spectra a. Hot Solids
2. Continuous Spectra b. Hot Stars
3. Absorption Spectra c. Hot Gases

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

83. 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

85. 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.

87. 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

94. Rhodamine 6G dye (tunable)
Laser Type
Wavelength (nm)
Argon fluoride (UV)
193
Krypton fluoride (UV)
248
Nitrogen (UV)
337
Argon (blue)
488
Argon (green)
514
Helium neon (green)
543
Helium neon (red)
633
Rhodamine 6G dye (tunable)
Ruby (CrAlO3) (red)
694
Nd:Yag (NIR)
1064
Carbon dioxide (FIR)
10600

95. 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

96. 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

97. Color
Chapter 27

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

99. Selective Reflection and Transmission
At an interface, light can be...
absorbed
reflected
transmitted

100. Fill in the Blanks absorb Black objects _______ all colors of light.
White objects _______ all colors of light.
Transparent objects _______ all colors of light
absorb
reflect
transmit

101. Color Filters
Red filters transmit red light and absorb the other colors, etc.
Red objects reflect red light and absorb the rest, etc.

103. In red light, what color do the red petals and green leaves of a rose appear?
Answer: Petal appear red
Leaves appear to be black.

104. 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.

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

108. 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.

111. Green Pigment Red Pigment
Absorbed =

112. Colors of Light

113. Yellow Pigment Cyan Pigment+ +
Absorbed =

114. Yellow + Cyan = Green Pigment
Absorbed =

115. In-Class Exercise
Make a list of the seven forms of light in order of decreasing wavelength.
Draw a Color Addition Diagram using overlapping colored spotlights. Label all 7 colors.
Draw a Color Subtraction Diagram using overlapping paints. Label all 7 colors.

117. Colors of Light
red
magenta
blue
yellow
white
cyan
green

118. yellow
green
cyan
red
black
blue
magenta

119. scatter high frequencies of light
Why is the Sky Blue?
Nitrogen and Oxygen
in our atmosphere
scatter high frequencies of light

120. Why is the Ocean Greenish Blue?
How have fish adapted?
Red light is absorbed by the molecules in the water

121. 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

125. 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

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

127. 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

136. 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 

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

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

139. 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 

140. 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. 

141. a) yellow, violet, and green. b) cyan, yellow, and magenta.
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. 

142. 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. 

143. Most of the light that we see has undergone
(a) selective interference
(b) selective transmission
(c) selective reflection
(d) selective refraction
(c) selective reflection

144. A mixture of magenta and green lights give white light
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

145. Mixing yellow paint and magenta paint gives what color?
(a) red
(b) green
(c) blue
(d) cyan

146. What color would red cloth appear if it were illuminated by cyan light?
(a) cyan
(b) red
(c) yellow
(d) green
(e) black 