Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Conceptual Physics
Fundamentals
Chapter 13:
LIGHT WAVES
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
This lecture will help you
understand:
• Electromagnetic Spectrum
• Transparent and Opaque Materials
• Color
• Why the Sky is Blue, Sunsets are Red,
and Clouds are White
• Diffraction
• Interference of Light
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Light Waves
“The light of stars that were extinguished
ages ago still reaches us. So it is with great
men who died centuries ago, but still reach
us with the radiations of their personalities.”
—Kahlil Gibran
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Light Waves
Light is the only thing we can see
• originates from the accelerated motion of electrons
• electromagnetic phenomenon
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Electromagnetic Spectrum
Electromagnetic wave
• made up of vibrating electric and magnetic
fields
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If an electron vibrates up and down 1000 times each
second, it generates an electromagnetic wave with a
A. period of 1000 seconds.
B. speed of 1000 m/s.
C. wavelength of 1000 m.
D. none of the above
Electromagnetic Spectrum
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If an electron vibrates up and down 1000 times each
second, it generates an electromagnetic wave with a
A. period of 1000 seconds.
B. speed of 1000 m/s.
C. wavelength of 1000 m.
D. none of the above
Explanation:
The vibrating electron would emit a wave with a frequency of 1000
Hz, which is not in the list above.
Electromagnetic Spectrum
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Electromagnetic Spectrum
Electromagnetic spectrum • classification of electromagnetic waves according to
frequency – lowest frequency of light we can see appears red
– highest frequency of light we can see appears violet
– higher frequency of light is ultraviolet—more energetic and causes sunburns
– beyond are X-ray and gamma ray
• no sharp boundary between regions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The electromagnetic spectrum spans waves ranging from
lowest to highest frequencies. The smallest portion of the
electromagnetic spectrum is that of
A. radio waves.
B. microwaves.
C. visible light.
D. gamma rays.
Electromagnetic Spectrum
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The electromagnetic spectrum spans waves ranging from
lowest to highest frequencies. The smallest portion of the
electromagnetic spectrum is that of
A. radio waves.
B. microwaves.
C. visible light.
D. gamma rays.
Electromagnetic Spectrum
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Which of these is fundamentally different from the others?
A. sound waves
B. light waves
C. radio waves
D. X-rays
Electromagnetic Spectrum
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Which of these is fundamentally different from the others?
A. sound waves
B. light waves
C. radio waves
D. X-rays
Explanation:
All are electromagnetic waves except sound, which is a
mechanical wave.
Electromagnetic Spectrum
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials Light is transmitted similar to sound
• light incident on matter forces some electrons in
matter to vibrate
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials How light penetrates transparent material
such as glass
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials How light penetrates transparent material
such as glass (continued)
• electrons or molecules in the glass are forced into
vibration
• energy is momentarily absorbed and vibrates the
electrons in the glass
• this vibrating electron either emits a photon or
transfers the energy as heat
• Time delay between absorption and reemission of
energy of vibrating electrons results in a lower average
speed of light through a transparent material
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials Average speed of light through different
materials • vacuum—c (300,000,000 m/s)
• atmosphere—slightly less than c (but rounded off to c)
• water—0.75 c
• glass—0.67 c, depending on material
• diamond—0.41 c
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Strictly speaking, the photons of light incident on glass are
A. also the ones that travel through and exit the other side.
B. not the ones that travel through and exit the other side.
C. absorbed and transformed to thermal energy.
D. diffracted.
Transparent and Opaque Materials
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Strictly speaking, the photons of light incident on glass are
A. also the ones that travel through and exit the other side.
B. not the ones that travel through and exit the other side.
C. absorbed and transformed to thermal energy.
D. diffracted.
Explanation:
Figure 13.6 illustrates this nicely. The light that exits the glass is not the same light
that begins the process of absorption and re-emission.
Transparent and Opaque Materials
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Compared with the frequency of illuminating light on a
sheet of transparent plastic, the frequency of light that is
transmitted
A. is slightly less.
B. is the same.
C. is slightly higher.
D. depends on the type of plastic.
Transparent and Opaque Materials
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Compared with the frequency of illuminating light on a
sheet of transparent plastic, the frequency of light that is
transmitted
A. is slightly less.
B. is the same.
C. is slightly higher.
D. depends on the type of plastic
Explanation:
Speed of light in plastic may vary, but the frequency transmitted doesn’t.
Transparent and Opaque Materials
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The average speed of light is less in
A. air before entering glass.
B. glass.
C. air after emerging from glass.
D. none of the above
Transparent and Opaque Materials
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The average speed of light is less in
A. air before entering glass.
B. glass.
C. air after emerging from glass.
D. none of the above
Transparent and Opaque Materials
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials Reflection
• Light shining on metal forces free electrons in
the metal into vibrations that emit their own light
as reflection.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Transparent and Opaque
Materials Light incident on:
• dry surfaces bounces directly to your eye
• wet surfaces bounces inside the transparent wet
region, absorbing energy with each bounce, and
reaches your eye darker than from a dry surface
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Color
• physiological experience
• in the eye of the beholder
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color Each color in a rainbow corresponds to a different
wavelength of electromagnetic spectrum.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Color we see depends on frequency of light
• lowest frequency—perceived as red
• in between lowest and highest frequency—
perceived as colors of the rainbow (red, orange,
yellow, green, blue, indigo, violet)
• highest frequency—perceived as violet
• beyond violet, invisible ultraviolet (UV)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Selective reflection
• We see the color of a rose by the light it
reflects.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Objects reflect light of some frequencies and
absorb the rest.
• rose petals absorb most of the light and reflect
red
• objects that absorb light and reflect none appear
black
• object can reflect only those frequencies present
in the illuminating light
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Color of transparent object depends on color of light it transmits.
• colored glass is warmed due to the energy of absorbed light illuminating the glass
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Mixed colored lights
• Distribution of solar frequencies is uneven – most intense in yellow-green portion (where our eyes
are most sensitive)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Radiation curve divides into three regions
that match the color receptors in our eyes.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Additive primary colors
• red, green, and blue
• produce any color in the spectrum
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Red, green, and blue light overlap to form
A. red light.
B. green light.
C. blue light.
D. white light.
Color
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Red, green, and blue light overlap to form
A. red light.
B. green light.
C. blue light.
D. white light.
Color
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When the color yellow is seen on your TV screen, the
phosphors being activated on the screen are
A. mainly yellow.
B. blue and red.
C. green and yellow.
D. red and green.
Color
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
When the color yellow is seen on your TV screen, the
phosphors being activated on the screen are
A. mainly yellow.
B. blue and red.
C. green and yellow.
D. red and green.
Color
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A blue object will appear black when illuminated with
A. blue light.
B. cyan light.
C. yellow light.
D. magenta light.
Color
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A blue object will appear black when illuminated with
A. blue light.
B. cyan light.
C. yellow light.
D. magenta light.
Color
CHECK YOUR ANSWER
Color subtraction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A red rose will not appear red when illuminated only with
A. red light.
B. orange light.
C. white light.
D. cyan light.
Color
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A red rose will not appear red when illuminated only with
A. red light.
B. orange light.
C. white light.
D. cyan light.
Color
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
3-D glasses use Cyan and Red
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color Printing
Computer monitors emit light, whereas inked paper
absorbs or reflects light
• RGB works best for emitting colored light
• CMY works best for reflecting colored light
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color Printing
Only three colors of ink (plus black) are used to print color photographs—(a)
magenta, (b) yellow, (c) cyan, which when combined produce the colors shown in
(d). The addition of black (e) produces the finished result (f).
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Why the Sky is Blue
Why the sky is blue • results of selective scattering of smaller particles
than the wavelength of incident light and resonances at frequencies higher than scattered light
• the tinier the particle, the higher the frequency of light it will reemit
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Why the Sky is Blue
Why the sky is blue (continued) • due to selective scattering
• blue scattered light predominates in our vision
• varies in different locations under various conditions
– clear dry day—much deeper blue sky
– clear, humid day—beautiful blue sky
– lots of dust particles and larger molecules than
nitrogen and oxygen in the atmosphere—less blue
sky with whitish appearance
– after heavy rainstorm (washing away of airborne
particles)—deeper blue sky
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Why Sunsets are Red
Light that is least scattered is light of low
frequencies, which best travel through air. • red
• orange
• yellow
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A variety of sunset colors is evidence for a variety of
A. elements in the Sun.
B. apparent atmosphere thickness.
C. atmospheric particles.
D. primary colors.
Why Sunsets are Red
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A variety of sunset colors is evidence for a variety of
A. elements in the Sun.
B. apparent atmosphere thickness.
C. atmospheric particles.
D. primary colors.
Why Sunsets are Red
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If molecules in the sky scattered orange light instead of
blue light, sunsets would be
A. orange.
B. yellow.
C. green.
D. blue.
Why Sunsets are Red
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If molecules in the sky scattered orange light instead of
blue light, sunsets would be
A. orange.
B. yellow.
C. green.
D. blue.
Explanation:
Of the colors listed, blue is closest to being the complementary color of orange.
Why Sunsets are Red
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Why Clouds are White
Clouds • clusters of various sizes of water droplets
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Why Clouds are White
Size of clusters determines scattered
cloud color • tiny clusters produce bluish clouds
• slightly large clusters produce greenish clouds
• larger clusters produce reddish clouds
• overall result is white clouds
• slightly larger clusters produce a deep grey
• still larger clusters produce raindrops
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Diffraction
• bending of waves by means other than reflection
and refraction
• property of all kinds of waves
• seen around edges of many shadows
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Waves diffract after passing through a
narrow opening.
Plane waves passing
through openings of
various sizes. The
smaller the opening,
the greater the
bending of the waves
at the edges.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Amount of diffraction depends on wavelength of
the wave compared to the size of the obstruction
that casts the shadow.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Features of diffraction
• limitations with focusing images in optical
instruments
– object about the same size as wavelength of light,
diffraction blurs
– object smaller than wavelength of light, no image
• limitations avoided with an electron beam having
extremely short wavelengths
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Features of diffraction (continued)
• electron microscopes use electric and magnetic
fields to focus and magnify images
• better radio reception with long radio waves
• for dolphins, use of shorter wavelengths see
finer detail—ultrasound
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference of Light
Superposition of waves
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference of Light
Interference pattern
• caused by interference between a pair of
waves
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference of Light
Interference pattern (continued)
• constructive interference produces bright
region where waves reinforce each other
(waves arriving in phase)
• destructive interference produces dark region
where waves cancel each other (waves
arriving a half wavelength out of phase)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference of Light
Detail of interference pattern
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The phenomenon of interference occurs for
A. sound waves.
B. light waves.
C. both A and B
D. neither A nor B
Interference of Light
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The phenomenon of interference occurs for
A. sound waves.
B. light waves.
C. both A and B
D. neither A nor B
Explanation:
Interference is the property that characterizes waves in general.
Interference of Light
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Diffraction
Diffraction grating
• composed of a large number of close, equally
spaced slits for analyzing light source
• produced by spectrometers that disperse white
light into colors
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference
Interference colors by reflection from thin
films
• The thin film of gasoline is just the right thickness to
result in the destructive interference of blue light.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If the thin film of gasoline was a bit thinner, the wavelength
to be cancelled would be
A. shorter than that of blue.
B. longer than that of blue.
C. white.
D. none of the above
Interference of Light
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If the thin film of gasoline was a bit thinner, the wavelength
to be cancelled would be
A. shorter than that of blue.
B. longer than that of blue.
C. white.
D. none of the above
Interference of Light
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If violet light were cancelled by the double reflection of
sunlight from gasoline on a wet surface, the resulting color
would likely be
A. red.
B. orange.
C. green.
D. violet.
Interference of Light
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If violet light were cancelled by the double reflection of
sunlight from gasoline on a wet surface, the resulting color
would likely be
A. red.
B. orange.
C. green.
D. violet.
Explanation:
Orange is the complementary color of violet.
Interference of Light
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If you see the color blue reflected in the interference from
gasoline on water, and you lower your head so a greater
angle from the normal results, you’ll likely see a color
having a wavelength
A. shorter than that of blue.
B. longer than that of blue.
C. with a white appearance.
D. none of the above
Interference of Light
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If you see the color blue reflected in the interference from
gasoline on water, and you lower your head so a greater
angle from the normal results, you’ll likely see a color
having a wavelength
A. shorter than that of blue.
B. longer than that of blue.
C. with a white appearance.
D. none of the above
Explanation:
The path through the gasoline would be longer, and a longer wavelength
would be cancelled. The result of a long wave being cancelled is a
shorter wave.
Interference of Light
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
If you see the color blue reflected in the interference from
gasoline on water, and you lower your head so a greater
angle from the normal results, you’ll likely see a color
having a wavelength
A. shorter than that of blue.
B. longer than that of blue.
C. with a white appearance.
D. none of the above
Explanation:
The path through the gasoline would be longer, and a longer wavelength
would be cancelled. The result of a long wave being cancelled is a
shorter wave.
Interference of Light
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Interference of Light
Interference colors
• Note the colors in the bubble are subtractive
primaries—magentas, yellows, and cyans.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
What can the human eye not see?
A. infrared radiation
B. ultraviolet radiation
C. both A and B
D. neither A nor B
Color
CHECK YOUR NEIGHBOR
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
What can the human eye not see?
A. infrared radiation
B. ultraviolet radiation
C. both A and B
D. neither A nor B
Color
CHECK YOUR ANSWER
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Subtractive primary colors
• combination of two of the three additive
primary colors
– red + blue = magenta
– red + green = yellow
– blue + green = cyan
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
The shadows of the golf ball are subtractive
• Magenta (opposite of green)
• Cyan (opposite of red)
• Yellow (opposite of blue)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Color
Subtractive primaries are complementary to additive primaries. • magenta + green = white = red + blue + green
• yellow + blue = white + red + green + blue
example: color printing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
A white sky is evidence that the atmosphere contains
A. predominantly small particles.
B. predominantly large particles.
C. a mixture of particle sizes.
D. pollutants.
Why the Sky is Blue
CHECK YOUR NEIGHBOR