chapter 18: the electromagnetic spectrum and light 18.1 electromagnetic waves

Chapter 18: The Chapter 18: The Electromagnetic Spectrum Electromagnetic Spectrum

and Lightand Light

18.1 18.1 Electromagnetic Electromagnetic

WavesWaves

Electromagnetic wavesElectromagnetic waves are are produced when an electric produced when an electric charge vibrates or charge vibrates or accelerates.accelerates.

Electromagnetic waves can Electromagnetic waves can travel through a vacuum, or travel through a vacuum, or empty space, as well as empty space, as well as through matter.through matter.

Electromagnetic wavesElectromagnetic waves are are transverse waves consisting of transverse waves consisting of changing electric fields and changing electric fields and changing magnetic fields.changing magnetic fields.

•Like mechanical waves, they Like mechanical waves, they carry energy from place to carry energy from place to place. place.

•Electromagnetic waves differ Electromagnetic waves differ from mechanical waves in from mechanical waves in how they are produced and how they are produced and how they travel.how they travel.

How They Are ProducedHow They Are Produced

Electromagnetic waves are produced by Electromagnetic waves are produced by constantly changing electric fields and constantly changing electric fields and magnetic fields. magnetic fields.

• An An electric fieldelectric field exerts electric forces exerts electric forces on charged particles. on charged particles. •Electric fields are produced by electrically Electric fields are produced by electrically

charged particles and by changing charged particles and by changing magnetic fields. magnetic fields.

• A A magnetic fieldmagnetic field produces magnetic produces magnetic forces.forces.•Magnetic fields are produced by magnets, Magnetic fields are produced by magnets,

by changing electric fields, and by by changing electric fields, and by vibrating charges. vibrating charges.

Electromagnetic waves are Electromagnetic waves are transverse waves because the transverse waves because the fields are at right angles to the fields are at right angles to the direction in which the wave direction in which the wave travels. travels.

How They TravelHow They Travel

Changing electric fields produce Changing electric fields produce changing magnetic fields, and changing magnetic fields, and changing magnetic fields produce changing magnetic fields produce changing electric fields, so the changing electric fields, so the fields regenerate each other.fields regenerate each other.

•Electromagnetic waves Electromagnetic waves do not do not need a medium.need a medium.

•The transfer of energy by The transfer of energy by electromagnetic waves traveling electromagnetic waves traveling through matter or across space through matter or across space is called is called electromagnetic electromagnetic radiation.radiation.

The Speed of LightThe Speed of Light

All electromagnetic waves All electromagnetic waves travel at the same speed travel at the same speed when in a vacuum, when in a vacuum, regardless of the regardless of the observer’s motion.observer’s motion.

The speed of light in a The speed of light in a vacuum, vacuum, c, c, is 3.00 × 10is 3.00 × 1088 meters per second.meters per second.

The speed of an electromagnetic The speed of an electromagnetic wave is the product of its wave is the product of its wavelength and its frequency. wavelength and its frequency.

•The speed of electromagnetic The speed of electromagnetic waves in a vacuum is waves in a vacuum is constant, so the wavelength is constant, so the wavelength is inversely proportional to the inversely proportional to the frequency. frequency.

•As the wavelength increases, As the wavelength increases, the frequency decreases.the frequency decreases.

Electromagnetic radiation Electromagnetic radiation behaves sometimes like a behaves sometimes like a wave and sometimes like a wave and sometimes like a stream of particles.stream of particles.

The emission of electrons from a The emission of electrons from a metal caused by light striking the metal caused by light striking the metal is called the metal is called the photoelectric photoelectric effecteffect. . In 1905, Albert Einstein (1879–1955) In 1905, Albert Einstein (1879–1955) proposed that light, and all proposed that light, and all electromagnetic radiation, consists of electromagnetic radiation, consists of packets of energy.packets of energy.

These packets of electromagnetic These packets of electromagnetic energy are now called energy are now called photonsphotons..

Each photon’s energy is Each photon’s energy is proportional to the frequency of proportional to the frequency of the light. the light.

Blue light has a higher Blue light has a higher frequency than red light, so frequency than red light, so photons of blue light have more photons of blue light have more energy than photons of red energy than photons of red light. light.

The intensity of light The intensity of light decreases as photons decreases as photons travel farther from the travel farther from the source.source.

IntensityIntensity is the rate at which a is the rate at which a wave’s energy flows through a wave’s energy flows through a given unit of area. given unit of area.

•As waves travel away from As waves travel away from the source, they pass through the source, they pass through a larger and larger area. a larger and larger area.

•The total energy does not The total energy does not change, so the wave’s change, so the wave’s intensity decreases.intensity decreases.

The closer you are to a surface when The closer you are to a surface when you spray paint it, the smaller the you spray paint it, the smaller the area the paint covers, and the more area the paint covers, and the more intense the paint color looks. intense the paint color looks.

Intensity

Assessment Questions

1.1. How are electromagnetic waves How are electromagnetic waves different from all mechanical waves? different from all mechanical waves? a.a. Electromagnetic waves don’t carry Electromagnetic waves don’t carry

energy.energy.b.b. Electromagnetic waves are invisible.Electromagnetic waves are invisible.c.c. Electromagnetic waves are Electromagnetic waves are

longitudinal waves.longitudinal waves.d.d. Electromagnetic waves can travel Electromagnetic waves can travel

through a vacuum.through a vacuum.


1.1. How are electromagnetic waves How are electromagnetic waves different from all mechanical waves? different from all mechanical waves? a.a. Electromagnetic waves don’t carry Electromagnetic waves don’t carry

energy.energy.b.b. Electromagnetic waves are invisible.Electromagnetic waves are invisible.c.c. Electromagnetic waves are Electromagnetic waves are

longitudinal waves.longitudinal waves.d.d.Electromagnetic waves can Electromagnetic waves can

travel through a vacuumtravel through a vacuum..


2.2.What is the wavelength of a radio What is the wavelength of a radio wave that has a frequency of 1.5 wave that has a frequency of 1.5 x 10x 1066 Hz? (c = 3.0x10 Hz? (c = 3.0x1088 m/s) m/s)a.a.45 m45 mb.b.200 m200 mc.c. 450 m450 md.d.2 km2 km


2.2.What is the wavelength of a radio What is the wavelength of a radio wave that has a frequency of 1.5 wave that has a frequency of 1.5 x 10x 1066 Hz? (c = 3.0x10 Hz? (c = 3.0x1088 m/s) m/s)a.a.45 m45 mb.b.200 m200 mc.c. 450 m450 md.d.2 km wave speed = 2 km wave speed = λλff


3.3.The photoelectric effect is The photoelectric effect is evidence that light behaves like evidence that light behaves like

a.a.a wave.a wave.

b.b.a particle.a particle.

c.c. both a wave and a particle.both a wave and a particle.

d.d.neither a wave nor a particle.neither a wave nor a particle.


4.4. As photons travel farther from a As photons travel farther from a light source, the intensity of light light source, the intensity of light stays the same. stays the same.

TrueTrueFalseFalse


and Lightand Light

18.2 The 18.2 The Electromagnetic Electromagnetic

SpectrumSpectrum

The full range of frequencies of The full range of frequencies of electromagnetic radiation is called electromagnetic radiation is called the the electromagnetic spectrumelectromagnetic spectrum..

•Visible light is the only part of the Visible light is the only part of the electromagnetic spectrum that electromagnetic spectrum that you can see, but it is just a small you can see, but it is just a small part.part.

•Each kind of wave is characterized Each kind of wave is characterized by a range of wavelengths and by a range of wavelengths and frequencies. All of these waves frequencies. All of these waves have many useful applications.have many useful applications.

http://lectureonline.cl.msu.edu/~mmp/applist/Spectrum/s.htm

Radio wavesRadio waves have the longest have the longest wavelengths in the wavelengths in the electromagnetic spectrum. electromagnetic spectrum.

Radio waves also have the Radio waves also have the lowest frequencies.lowest frequencies.

MicrowavesMicrowaves

The shortest-wavelength radio The shortest-wavelength radio waves are called waves are called microwavesmicrowaves. .

Microwaves cook and reheat Microwaves cook and reheat food. Microwaves also carry cell food. Microwaves also carry cell phone conversations. The phone conversations. The process works much like a radio process works much like a radio broadcast.broadcast.

Radar Radar (ra(radio dio ddetection etection aand nd rranging)anging)

Radar technology uses a radio Radar technology uses a radio transmitter to send out short transmitter to send out short bursts of radio waves. bursts of radio waves.

•They reflect off the objects They reflect off the objects they encounter and bounce they encounter and bounce back toward where they came back toward where they came from. from.

•Returning waves are then Returning waves are then picked up by a radio receiver.picked up by a radio receiver.

InfraredInfrared rays have higher rays have higher frequencies than radio waves frequencies than radio waves and lower frequencies than red and lower frequencies than red light. Your skin senses infrared light. Your skin senses infrared radiation as warmth. radiation as warmth. Restaurants use infrared lamps Restaurants use infrared lamps to keep foods warm.to keep foods warm.

Warmer objects give off more Warmer objects give off more infrared radiation than cooler infrared radiation than cooler objects. objects.

A device called a A device called a thermographthermograph uses infrared sensors to create uses infrared sensors to create thermogramsthermograms, color-coded , color-coded pictures that show variations in pictures that show variations in temperature. temperature.

The visible part of the The visible part of the electromagnetic spectrum is electromagnetic spectrum is light that the human eye can light that the human eye can see. see.

Each wavelength in the Each wavelength in the visible spectrum visible spectrum corresponds to a specific corresponds to a specific frequency and has a frequency and has a particular color. particular color.

Ultraviolet rays have shorter Ultraviolet rays have shorter wavelengths and higher wavelengths and higher frequenciesfrequencies

•Some exposure to ultraviolet Some exposure to ultraviolet rays helps your skin produce rays helps your skin produce vitamin D, which helps the vitamin D, which helps the body absorb calcium from body absorb calcium from foods. foods.

•Excessive exposure can cause Excessive exposure can cause sunburn, wrinkles, skin sunburn, wrinkles, skin cancer, and eye damage.cancer, and eye damage.

•Ultraviolet rays are used to Ultraviolet rays are used to kill microorganisms. In kill microorganisms. In winter, plant nurseries use winter, plant nurseries use ultraviolet lights to help ultraviolet lights to help plants grow.plants grow.

X-rays X-rays have very short have very short wavelengthswavelengths

X-rays have high energy and X-rays have high energy and can penetrate matter that can penetrate matter that light cannot.light cannot.

Too much exposure to X-rays Too much exposure to X-rays can kill or damage living can kill or damage living tissue.tissue.

Gamma raysGamma rays have the shortest have the shortest wavelengths in the wavelengths in the electromagnetic spectrum. electromagnetic spectrum.

They have the highest They have the highest frequencies, the most energy, frequencies, the most energy, and the greatest penetrating and the greatest penetrating ability of all the electromagnetic ability of all the electromagnetic waves. waves.

Exposure to tiny amounts of Exposure to tiny amounts of gamma rays is tolerable, but gamma rays is tolerable, but overexposure can be deadly.overexposure can be deadly.

• Gamma rays are used in Gamma rays are used in radiation therapy to kill radiation therapy to kill cancer cells without cancer cells without harming nearby healthy harming nearby healthy cells. cells.

• Gamma rays are also Gamma rays are also used to make pictures of used to make pictures of the human brain, with the human brain, with different levels of brain different levels of brain activity represented by activity represented by different colors.different colors.

• Pipelines are checked Pipelines are checked with machines that with machines that travel on the inside of a travel on the inside of a pipe, taking gamma ray pipe, taking gamma ray pictures along the entire pictures along the entire length. length.

Radiation treatment using radioactive cobalt-

60.


1.1.Which waves have the longest Which waves have the longest wavelength? wavelength?

a.a.radio wavesradio waves

b.b.infrared raysinfrared rays

c.c. visible lightvisible light

d.d.ultraviolet raysultraviolet rays


2.2.What type of electromagnetic What type of electromagnetic radiation is used to keep prepared radiation is used to keep prepared foods warm in a serving area?foods warm in a serving area?a.a.ultraviolet raysultraviolet raysb.b.infrared raysinfrared raysc.c. X-raysX-raysd.d.gamma raysgamma rays


and Lightand Light

18.3 Behavior of 18.3 Behavior of LightLight

A A transparent materialtransparent material transmits light, which transmits light, which means it allows most of means it allows most of the light that strikes it to the light that strikes it to pass through it. pass through it.

A A translucent materialtranslucent material scatters light. If you can scatters light. If you can see through a material, see through a material, but the objects you see but the objects you see through it do not look through it do not look clear or distinct, then the clear or distinct, then the material is translucent.material is translucent.

An An opaque materialopaque material either either absorbs or reflects all of the absorbs or reflects all of the light that strikes it. Most light that strikes it. Most materials are opaque.materials are opaque.

An opaque object does not An opaque object does not allow any light to pass allow any light to pass through it. through it.

When light strikes a new When light strikes a new medium, the light can be medium, the light can be reflected, absorbed, or reflected, absorbed, or transmitted. When light is transmitted. When light is transmitted, it can be transmitted, it can be refracted, polarized, or refracted, polarized, or scattered.scattered.

ReflectionReflectionAn An imageimage is a copy of an object is a copy of an object formed by reflected (or refracted) formed by reflected (or refracted) waves of light. waves of light.

Regular reflectionRegular reflection occurs when occurs when parallel light waves strike a surface parallel light waves strike a surface and reflect all in the same direction. and reflect all in the same direction.

Diffuse reflectionDiffuse reflection occurs when occurs when parallel light waves strike a rough, parallel light waves strike a rough, uneven surface and reflect in many uneven surface and reflect in many different directions.different directions.

Law of ReflectionLaw of Reflection

•Angle of incidenceAngle of incidence (angle of (angle of incoming light rays) equals incoming light rays) equals the the angle of reflectionangle of reflection (angle (angle of reflected light rays)of reflected light rays)

RefractionRefractionA light wave can refract, or A light wave can refract, or bend, when it passes at an angle bend, when it passes at an angle from one medium into another.from one medium into another.

Refraction makes underwater Refraction makes underwater objects appear closer and larger objects appear closer and larger than they really are. than they really are.

Refraction can also sometimes Refraction can also sometimes cause a cause a mirage,mirage, a false or a false or distorted image.distorted image.

•occurs because light travels occurs because light travels faster in hot air than in faster in hot air than in cooler, denser air. cooler, denser air.

PolarizationPolarizationLight with waves that vibrate in only Light with waves that vibrate in only one plane is one plane is polarized lightpolarized light. .

•Light reflecting from a nonmetallic flat Light reflecting from a nonmetallic flat surface, such as a window or the surface of surface, such as a window or the surface of a lake, can become polarized. a lake, can become polarized.

•Horizontally polarized light reflects more Horizontally polarized light reflects more strongly than the rest of the sunlight. This strongly than the rest of the sunlight. This reflection produces glare. reflection produces glare.

•Polarized sunglasses have vertically Polarized sunglasses have vertically polarized filters to block the horizontally polarized filters to block the horizontally polarized light.polarized light.

ScatteringScatteringIn In scatteringscattering, light is redirected as it , light is redirected as it passes through a medium. passes through a medium.

•Most of the particles in the Most of the particles in the atmosphere are very small. Small atmosphere are very small. Small particles scatter shorter-particles scatter shorter-wavelength blue light more than wavelength blue light more than light of longer wavelengths.light of longer wavelengths.

Blue light is scattered in all directions more than other colors of light, which makes the sky appear blue.

Assessment Questions1.1.How do polarized sunglasses How do polarized sunglasses

reduce glare? reduce glare?

a.a.by scattering light as it passes by scattering light as it passes through the glassesthrough the glasses

b.b.by providing a smooth surface by providing a smooth surface that light can reflect off that light can reflect off

c.c. by absorbing all lightby absorbing all light

d.d.by blocking horizontally by blocking horizontally polarized lightpolarized light


2.2. An opaque material passes light An opaque material passes light through but scatters the light so through but scatters the light so that objects do not look clear. that objects do not look clear.

TrueTrueFalseFalse


and Lightand Light

18.4 Color18.4 Color

Sunlight is made up of all the Sunlight is made up of all the colors of the visible spectrum. A colors of the visible spectrum. A prismprism separates white light into separates white light into a visible spectrum. a visible spectrum.

•When red light, with its longer When red light, with its longer wavelength, enters a glass wavelength, enters a glass prism, it slows down the least prism, it slows down the least of all the colors.of all the colors.

•Red light is bent the least. Red light is bent the least. •Violet light is bent the most.Violet light is bent the most.

The process in which white light The process in which white light separates into colors is called separates into colors is called dispersiondispersion. A rainbow forms . A rainbow forms when droplets of water in the air when droplets of water in the air act like prisms.act like prisms.

The color of any object The color of any object depends on what the depends on what the object is made of and object is made of and on the color of light on the color of light that strikes the object.that strikes the object.

An object’s color is the An object’s color is the color of light that color of light that reaches your eye when reaches your eye when you look at the object. you look at the object.

•Sunlight contains all the Sunlight contains all the colors of the visible colors of the visible spectrum. spectrum.

•A red car in sunlight A red car in sunlight reflects mostly red light. reflects mostly red light.

•Most of the rest of the Most of the rest of the light is absorbed at the light is absorbed at the surface of the paintsurface of the paint..

Primary colors are three Primary colors are three specific colors that can be specific colors that can be combined in varying combined in varying amounts to create all amounts to create all possible colors.possible colors.The primary colors of light The primary colors of light are are redred,, greengreen,, andand blue. blue.

The three The three primary primary colors of colors of light are red, light are red, green, and green, and blue. When blue. When any two any two primary primary colors colors combine, a combine, a secondary secondary color is color is formed.formed.

Mixing Colors of Light

If you add a primary color to the If you add a primary color to the proper secondary color, you will proper secondary color, you will get white light. get white light.

Two colors of light that combine Two colors of light that combine to form white light are to form white light are complementary colors of light. complementary colors of light.

A A complementary color paircomplementary color pair is a is a combination of one primary combination of one primary color and one secondary color. color and one secondary color.

Light QuestionLight Question•Why would a purple-people Why would a purple-people

eater appear black under eater appear black under yellow light?yellow light?

The primary colors of The primary colors of pigments are cyan, pigments are cyan, yellow, and magenta.yellow, and magenta.

A A pigmentpigment is a material that is a material that absorbs some colors of light absorbs some colors of light and reflects other colors. and reflects other colors.

The three The three primary colors primary colors of pigments of pigments are cyan, are cyan, yellow, and yellow, and magenta. magenta. When the When the three primary three primary colors of colors of pigments are pigments are combined, the combined, the secondary secondary colors of colors of pigments are pigments are formed. formed.

Mixing Pigments

Assessment Questions1.1. A prism separates white light into the A prism separates white light into the

visible spectrum because visible spectrum because

a.a. longer wavelengths are absorbed longer wavelengths are absorbed more than shorter wavelengths.more than shorter wavelengths.

b.b. shorter wavelengths refract more shorter wavelengths refract more than longer wavelengths.than longer wavelengths.

c.c. shorter wavelengths reflect more shorter wavelengths reflect more than longer wavelengths. than longer wavelengths.

d.d. longer wavelengths experience more longer wavelengths experience more interference.interference.

Assessment Questions1.1. A prism separates white light into the A prism separates white light into the

visible spectrum because visible spectrum because

a.a. longer wavelengths are absorbed longer wavelengths are absorbed more than shorter wavelengths.more than shorter wavelengths.

b.b.shorter wavelengths refract shorter wavelengths refract more than longer wavelengthsmore than longer wavelengths..

c.c. shorter wavelengths reflect more shorter wavelengths reflect more than longer wavelengths. than longer wavelengths.

d.d. longer wavelengths experience more longer wavelengths experience more interference.interference.


2.2. Which of these colors is one of the Which of these colors is one of the primary colors of light? primary colors of light?

a.a.greengreen

b.b.magentamagenta

c.c. yellowyellow

d.d.whitewhite


and Lightand Light

18.5 Sources of 18.5 Sources of LightLight

Objects that give off their own Objects that give off their own light are light are luminousluminous. . The sun is The sun is luminous, as are all light luminous, as are all light sources. sources.

The light produced when an The light produced when an object gets hot enough to object gets hot enough to glow is glow is incandescentincandescent. . The The filaments in incandescent light filaments in incandescent light bulbs are made of a substance bulbs are made of a substance called tungsten. (Old light called tungsten. (Old light bulbs)bulbs)

Incandescent bulbs give off Incandescent bulbs give off most of their energy as heat, most of their energy as heat, not light.not light.http://www.edisontechcenter.org/http://www.edisontechcenter.org/incandescent.htmlincandescent.html

In a process called In a process called fluorescencefluorescence, , a material absorbs light at one a material absorbs light at one wavelength and then emits light wavelength and then emits light at a longer wavelength. at a longer wavelength.

A A phosphorphosphor is a solid material is a solid material that can emit light by that can emit light by fluorescence. fluorescence.

A fluorescent bulb is a glass tube, A fluorescent bulb is a glass tube, containing mercury vapor, that is containing mercury vapor, that is coated with phosphors. (over your coated with phosphors. (over your head now)head now)http://www.edisontechcenter.org/http://www.edisontechcenter.org/Fluorescent.htmlFluorescent.html

Light in which waves have the same Light in which waves have the same wavelength, and the crests and wavelength, and the crests and troughs are lined up, is troughs are lined up, is coherent coherent lightlight. .

• A A laserlaser is a device that generates is a device that generates a beam of coherent light.a beam of coherent light.

• http://www.engineerguy.com/http://www.engineerguy.com/elements/videos/video-laser.htmelements/videos/video-laser.htm

Neon lightsNeon lights emit emit light when electrons light when electrons move through a gas move through a gas or a mixture of or a mixture of gases inside glass gases inside glass tubing.tubing. http://http://www.edisontechcenter.orwww.edisontechcenter.org/NeonLamps.htmlg/NeonLamps.html

Sodium-vapor lightsSodium-vapor lights contain a contain a small amount of solid sodium, in a small amount of solid sodium, in a mixture of neon and argon gases. mixture of neon and argon gases. (street lights)(street lights)

The current of electrons knocks The current of electrons knocks electrons in sodium to higher electrons in sodium to higher energy levels. When the electrons energy levels. When the electrons move back to lower energy levels, move back to lower energy levels, the sodium atoms emit light. the sodium atoms emit light. http://www.edisontechcenter.org/http://www.edisontechcenter.org/SodiumLamps.htmlSodiumLamps.html

Inside a tungsten-halogen Inside a tungsten-halogen bulb, electrons flow bulb, electrons flow through a tungsten through a tungsten filament. The filament filament. The filament gets hot and emits light.gets hot and emits light. (Automobile headlight bulbs)(Automobile headlight bulbs)http://www.edisontechcenter.org/http://www.edisontechcenter.org/halogen.htmlhalogen.html


1.1.The light produced when an object The light produced when an object becomes hot enough to glow is becomes hot enough to glow is

a.a. incandescent.incandescent.

b.b.fluorescent.fluorescent.

c.c. phosphorescent.phosphorescent.

d.d.coherent.coherent.


1.1.The light produced when an object The light produced when an object becomes hot enough to glow is becomes hot enough to glow is

a.a.incandescent.incandescent.

b.b.fluorescent.fluorescent.

c.c. phosphorescent.phosphorescent.

d.d.coherent.coherent.


2.2.The most efficient source of The most efficient source of lighting rooms of a building islighting rooms of a building isa.a. incandescent light.incandescent light.b.b.fluorescent light.fluorescent light.c.c. sodium-vapor light.sodium-vapor light.d.d.tungsten-halogen light.tungsten-halogen light.

chapter 18: the electromagnetic spectrum and light 18.1 electromagnetic waves

Documents

speed of electromagnetic

electromagnetic waves

mechanical waves

transverse waves

longitudinal waves

producedelectromagnetic

electromagnetic radiation

waves energy flows