Conceptual Physics Fundamentalssrjcstaff.santarosa.edu/~alee3/Physics 11/Powerpoint... · 2012. 12. 8. · Conceptual Physics Fundamentals Chapter 13: LIGHT WAVES . ... The vibrating

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

Conceptual Physics

Fundamentals

Chapter 13:

LIGHT WAVES


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


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


Light Waves

Light is the only thing we can see

• originates from the accelerated motion of electrons

• electromagnetic phenomenon


Electromagnetic Spectrum

Electromagnetic wave

• made up of vibrating electric and magnetic

fields


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


CHECK YOUR NEIGHBOR


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.


Explanation:

The vibrating electron would emit a wave with a frequency of 1000

Hz, which is not in the list above.


CHECK YOUR ANSWER



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


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.


CHECK YOUR NEIGHBOR


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.


CHECK YOUR ANSWER


Which of these is fundamentally different from the others?

A. sound waves

B. light waves

C. radio waves

D. X-rays


CHECK YOUR NEIGHBOR


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.


CHECK YOUR ANSWER


Transparent and Opaque

Materials Light is transmitted similar to sound

• light incident on matter forces some electrons in

matter to vibrate



Materials How light penetrates transparent material

such as glass



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



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


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


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.


CHECK YOUR ANSWER


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.


CHECK YOUR NEIGHBOR


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.


CHECK YOUR ANSWER


The average speed of light is less in

A. air before entering glass.

B. glass.

C. air after emerging from glass.



CHECK YOUR NEIGHBOR


The average speed of light is less in

A. air before entering glass.

B. glass.

C. air after emerging from glass.



CHECK YOUR ANSWER


Atmospheric Window




Materials Reflection

• Light shining on metal forces free electrons in

the metal into vibrations that emit their own light

as reflection.



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


Color

Color

• physiological experience

• in the eye of the beholder


Color Each color in a rainbow corresponds to a different

wavelength of electromagnetic spectrum.


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)


Color

Selective reflection

• We see the color of a rose by the light it

reflects.


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


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


Color

Mixed colored lights

• Distribution of solar frequencies is uneven – most intense in yellow-green portion (where our eyes

are most sensitive)


Color

Radiation curve divides into three regions

that match the color receptors in our eyes.


Color

Additive primary colors

• red, green, and blue

• produce any color in the spectrum

http://www.youtube.com/watch?v=xzmXrC-Yzfc&feature=related

http://www.physicsclassroom.com/class/light/u12l2d.cfm


Red, green, and blue light overlap to form

A. red light.

B. green light.

C. blue light.

D. white light.

Color

CHECK YOUR NEIGHBOR


Red, green, and blue light overlap to form

A. red light.

B. green light.

C. blue light.

D. white light.

Color

CHECK YOUR ANSWER


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


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


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


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

http://www.physicsclassroom.com/class/light/U12L2e.cfm


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


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


3-D glasses use Cyan and Red


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


Color Printing


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).


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


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


Why Sunsets are Red

Light that is least scattered is light of low

frequencies, which best travel through air. • red

• orange

• yellow


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


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


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


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


Why Clouds are White

Clouds • clusters of various sizes of water droplets


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


Diffraction

Diffraction

• bending of waves by means other than reflection

and refraction

• property of all kinds of waves

• seen around edges of many shadows


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.


Diffraction

Amount of diffraction depends on wavelength of

the wave compared to the size of the obstruction

that casts the shadow.


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


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


Interference of Light

Superposition of waves



Interference pattern

• caused by interference between a pair of

waves



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)



Detail of interference pattern


The phenomenon of interference occurs for

A. sound waves.

B. light waves.

C. both A and B

D. neither A nor B


CHECK YOUR NEIGHBOR


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.


CHECK YOUR ANSWER


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


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.


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.



CHECK YOUR NEIGHBOR


If the thin film of gasoline was a bit thinner, the wavelength

to be cancelled would be



C. white.



CHECK YOUR ANSWER


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.


CHECK YOUR NEIGHBOR


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.


CHECK YOUR ANSWER



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



C. with a white appearance.



CHECK YOUR NEIGHBOR





having a wavelength





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.


CHECK YOUR ANSWER






having a wavelength





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.


CHECK YOUR ANSWER



Interference colors

• Note the colors in the bubble are subtractive

primaries—magentas, yellows, and cyans.


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


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


• BACKUP


Color

Subtractive primary colors

• combination of two of the three additive

primary colors

– red + blue = magenta

– red + green = yellow

– blue + green = cyan


Color

The shadows of the golf ball are subtractive

• Magenta (opposite of green)

• Cyan (opposite of red)

• Yellow (opposite of blue)


Color

Subtractive primaries are complementary to additive primaries. • magenta + green = white = red + blue + green

• yellow + blue = white + red + green + blue

example: color printing


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


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 ANSWER

Documents

Conceptual Physics Fundamentalssrjcstaff.santarosa.edu/~alee3/Physics 11/Powerpoint... · 2012. 12. 8. · Conceptual Physics Fundamentals Chapter 13: LIGHT WAVES . ... The vibrating