Post on 01-Apr-2018
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Conceptual Physics Fundamentals
Chapter 13: LIGHT WAVES
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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
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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
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Light WavesLight is the only thing we can see• originates from the accelerated motion of
electrons• electromagnetic phenomenon
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Electromagnetic Spectrum
Electromagnetic wave• made up of vibrating electric and magnetic
fields
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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 SpectrumCHECK YOUR NEIGHBOR
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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 SpectrumCHECK YOUR ANSWER
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Electromagnetic SpectrumElectromagnetic 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
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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 SpectrumCHECK YOUR NEIGHBOR
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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 SpectrumCHECK YOUR ANSWER
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Which of these is fundamentally different from the others?
A. sound wavesB. light wavesC. radio wavesD. X-rays
Electromagnetic SpectrumCHECK YOUR NEIGHBOR
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Which of these is fundamentally different from the others?
A. sound wavesB. light wavesC. radio wavesD. X-rays
Explanation:All are electromagnetic waves except sound, which is a mechanical wave.
Electromagnetic SpectrumCHECK YOUR ANSWER
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Transparent and Opaque Materials
Light is transmitted similar to sound• light incident on matter forces some electrons in
matter to vibrate
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Transparent and Opaque Materials
How light penetrates transparent material such as glass
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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
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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
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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 MaterialsCHECK YOUR NEIGHBOR
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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 MaterialsCHECK YOUR ANSWER
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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 MaterialsCHECK YOUR NEIGHBOR
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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 MaterialsCHECK YOUR ANSWER
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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 MaterialsCHECK YOUR NEIGHBOR
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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 MaterialsCHECK YOUR ANSWER
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Transparent and Opaque Materials
Reflection• Light shining on metal forces free electrons in
the metal into vibrations that emit their own light as reflection.
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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
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Color
Color• physiological experience• in the eye of the beholder
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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)
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Color
Selective reflection• We see the color of a rose by the light it
reflects.
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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
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ColorColor of transparent object depends on color of light it transmits.
• colored glass is warmed due to the energy of absorbed light illuminating the glass
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Color
Mixed colored lights• Distribution of solar frequencies is uneven
– most intense in yellow-green portion (where our eyes are most sensitive)
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Color
Radiation curve divides into three regions that match the color receptors in our eyes.
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Color
Additive primary colors• red, green, and blue• produce any color in the spectrum
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Red, green, and blue light overlap to form
A. red light.B. green light.C. blue light.D. white light.
ColorCHECK YOUR NEIGHBOR
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Red, green, and blue light overlap to form
A. red light.B. green light.C. blue light.D. white light.
ColorCHECK YOUR ANSWER
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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.
ColorCHECK YOUR NEIGHBOR
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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.
ColorCHECK YOUR ANSWER
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A blue object will appear black when illuminated with
A. blue light.B. cyan light.C. yellow light.D. magenta light.
ColorCHECK YOUR NEIGHBOR
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A blue object will appear black when illuminated with
A. blue light.B. cyan light.C. yellow light.D. magenta light.
ColorCHECK YOUR ANSWER
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Color
Subtractive primary colors• combination of two of the three additive
primary colors– red + blue = magenta– red + green = yellow– blue + green = cyan
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Color
The shadows of the golf ball are subtractive• Magenta (opposite of green)• Cyan (opposite of red)• Yellow (opposite of blue)
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Color
Subtractive primaries are complementary to additive primaries.• magenta + green = white = red + blue + green• yellow + blue = white + red + green + blue
example: color printing
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A red rose will not appear red when illuminated only with
A. red light.B. orange light.C. white light.D. cyan light.
ColorCHECK YOUR NEIGHBOR
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A red rose will not appear red when illuminated only with
A. red light.B. orange light.C. white light.D. cyan light.
ColorCHECK YOUR ANSWER
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Color
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).
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Why the Sky is BlueWhy 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
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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
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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 BlueCHECK YOUR NEIGHBOR
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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 BlueCHECK YOUR ANSWER
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Why Sunsets are Red
Light that is least scattered is light of low frequencies, which best travel through air.• red• orange• yellow
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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 RedCHECK YOUR NEIGHBOR
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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 RedCHECK YOUR ANSWER
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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 RedCHECK YOUR NEIGHBOR
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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 RedCHECK YOUR ANSWER
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Why Clouds are White
Clouds• clusters of various sizes of water droplets
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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
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Diffraction
Diffraction• bending of waves by means other than reflection
and refraction• property of all kinds of waves• seen around edges of many shadows
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DiffractionWaves 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.
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Diffraction
Amount of diffraction depends on wavelength of the wave compared to the size of the obstruction that casts the shadow.
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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
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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
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Interference of Light
Superposition of waves
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Interference of Light
Interference pattern• caused by interference between a pair of
waves
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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)
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Interference of Light
Detail of interference pattern
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The phenomenon of interference occurs for
A. sound waves.B. light waves.C. both A and BD. neither A nor B
Interference of LightCHECK YOUR NEIGHBOR
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The phenomenon of interference occurs for
A. sound waves.B. light waves.C. both A and BD. neither A nor B
Explanation:Interference is the property that characterizes waves in general.
Interference of LightCHECK YOUR ANSWER
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DiffractionDiffraction grating• composed of a large number of close, equally
spaced slits for analyzing light source
• produced by spectrometers that disperse white light into colors
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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.
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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 LightCHECK YOUR NEIGHBOR
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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 LightCHECK YOUR ANSWER
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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 LightCHECK YOUR NEIGHBOR
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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 LightCHECK YOUR ANSWER
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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 LightCHECK YOUR NEIGHBOR
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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 LightCHECK YOUR ANSWER
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Interference of Light
Interference colors
• Note the colors in the bubble are subtractive primaries—magentas, yellows, and cyans.
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What can the human eye not see?
A. infrared radiationB. ultraviolet radiationC. both A and BD. neither A nor B
ColorCHECK YOUR NEIGHBOR
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What can the human eye not see?
A. infrared radiationB. ultraviolet radiationC. both A and BD. neither A nor B
ColorCHECK YOUR ANSWER