Chapter 18: The Electromagnetic Spectrum and Light

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Chapter 18 The Electromagnetic Spectrum and
Light

• 18.1 Electromagnetic Waves

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Electromagnetic waves are produced when an
electric charge vibrates or accelerates.
Electromagnetic waves can travel through a
vacuum, or empty space, as well as through
matter.
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• Electromagnetic waves are transverse waves
  consisting of changing electric fields and
  changing magnetic fields.
• Like mechanical waves, they carry energy from
  place to place.
• Electromagnetic waves differ from mechanical
  waves in how they are produced and how they
  travel.

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• How They Are Produced
• Electromagnetic waves are produced by constantly
  changing electric fields and magnetic fields.
• An electric field in a region of space exerts
  electric forces on charged particles. Electric
  fields are produced by electrically charged
  particles and by changing magnetic fields.
• A magnetic field in a region of space produces
  magnetic forces. Magnetic fields are produced by
  magnets, by changing electric fields, and by
  vibrating charges.

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

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• How They Travel
• Changing electric fields produce changing
  magnetic fields, and changing magnetic fields
  produce changing electric fields, so the fields
  regenerate each other.
• Electromagnetic waves do not need a medium.
• The transfer of energy by electromagnetic waves
  traveling through matter or across space is
  called electromagnetic radiation.

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• The Speed of Light
• All electromagnetic waves travel at the same
  speed when in a vacuum, regardless of the
  observers motion.
• The speed of light in a vacuum, c, is 3.00 108
  meters per second.

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• The speed of an electromagnetic wave is the
  product of its wavelength and its frequency.
• The speed of electromagnetic waves in a vacuum is
  constant, so the wavelength is inversely
  proportional to the frequency.
• As the wavelength increases, the frequency
  decreases.

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Electromagnetic radiation behaves sometimes like
a wave and sometimes like a stream of particles.
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• The emission of electrons from a metal caused by
  light striking the metal is called the
  photoelectric effect.
• In 1905, Albert Einstein (18791955) proposed
  that light, and all electromagnetic radiation,
  consists of packets of energy.
• These packets of electromagnetic energy are now
  called photons.

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• Each photons energy is proportional to the
  frequency of the light. Blue light has a higher
  frequency than red light, so photons of blue
  light have more energy than photons of red light.

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The intensity of light decreases as photons
travel farther from the source.
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• Intensity is the rate at which a waves energy
  flows through a given unit of area. A wave model
  also explains how intensity decreases.
• As waves travel away from the source, they pass
  through a larger and larger area.
• The total energy does not change, so the waves
  intensity decreases.

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Intensity

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

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Wave Connection

• Security systems consist of a beam of light
  shining on a metal plate. As electrons are
  knocked out, they create a current of
  electricity. If something crosses the light, the
  electricity stops and an alarms sounds. Why is
  the photoelectric effect necessary to explain
  this?

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Assessment Questions

• How are electromagnetic waves different from all
  mechanical waves?
• Electromagnetic waves dont carry energy.
• Electromagnetic waves are invisible.
• Electromagnetic waves are longitudinal waves.
• Electromagnetic waves can travel through a
  vacuum.

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Assessment Questions

• What is the wavelength of a radio wave that has a
  frequency of 1.5 x 106 Hz? (c 3.0x108 m/s)
• 45 m
• 200 m
• 450 m
• 2 km

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Assessment Questions

• The photoelectric effect is evidence that light
  behaves like
• a wave.
• a particle.
• both a wave and a particle.
• neither a wave nor a particle.

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Assessment Questions

• 4. As photons travel farther from a light source,
  the intensity of light stays the same.
  TrueFalse

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Chapter 18 The Electromagnetic Spectrum and Light

• 18.2 The Electromagnetic Spectrum

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The electromagnetic spectrum includes radio
waves, infrared rays, visible light, ultraviolet
rays, X-rays, and gamma rays.
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• The full range of frequencies of electromagnetic
  radiation is called the electromagnetic spectrum.
• Visible light is the only part of the
  electromagnetic spectrum that you can see, but it
  is just a small part.
• Each kind of wave is characterized by a range of
  wavelengths and frequencies. All of these waves
  have many useful applications.

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• Radio waves have the longest wavelengths in the
  electromagnetic spectrum.
• Radio waves also have the lowest frequencies.

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• Microwaves
• The shortest-wavelength radio waves are called
  microwaves.
• Microwaves cook and reheat food. Microwaves also
  carry cell phone conversations. The process works
  much like a radio broadcast.

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• Radar (radio detection and ranging)
• Radar technology uses a radio transmitter to send
  out short bursts of radio waves.
• They reflect off the objects they encounter and
  bounce back toward where they came from.
• Returning waves are then picked up by a radio
  receiver.

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

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• Warmer objects give off more infrared radiation
  than cooler objects.
• A device called a thermograph uses infrared
  sensors to create thermograms, color-coded
  pictures that show variations in temperature.

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• The visible part of the electromagnetic spectrum
  is light that the human eye can see.
• Each wavelength in the visible spectrum
  corresponds to a specific frequency and has a
  particular color.

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• Ultraviolet rays vary from about 400 nm to about
  4 nm.
• Some exposure to ultraviolet rays helps your skin
  produce vitamin D, which helps the body absorb
  calcium from foods.
• Excessive exposure can cause sunburn, wrinkles,
  skin cancer, and eye damage.
• Ultraviolet rays are used to kill microorganisms.
  In winter, plant nurseries use ultraviolet lights
  to help plants grow.

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• X-rays have very short wavelengths
• X-rays have high energy and can penetrate matter
  that light cannot.
• Too much exposure to X-rays can kill or damage
  living tissue.

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• Gamma rays have the shortest wavelengths in the
  electromagnetic spectrum.
• They have the highest frequencies, the most
  energy, and the greatest penetrating ability of
  all the electromagnetic waves.
• Exposure to tiny amounts of gamma rays is
  tolerable, but overexposure can be deadly.

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• Gamma rays are used in radiation therapy to kill
  cancer cells without harming nearby healthy
  cells.
• Gamma rays are also used to make pictures of the
  human brain, with different levels of brain
  activity represented by different colors.
• Pipelines are checked with machines that travel
  on the inside of a pipe, taking gamma ray
  pictures along the entire length.

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Assessment Questions

• Which waves have the longest wavelength?
• radio waves
• infrared rays
• visible light
• ultraviolet rays

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Assessment Questions

• What type of electromagnetic radiation is used to
  keep prepared foods warm in a serving area?
• ultraviolet rays
• infrared rays
• X-rays
• gamma rays

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Chapter 18 The Electromagnetic Spectrum and Light

• 18.3 Behavior of Light

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Materials can be transparent, translucent, or
opaque.
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• A transparent material transmits light, which
  means it allows most of the light that strikes it
  to pass through it.

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

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• An opaque material either absorbs or reflects all
  of the light that strikes it. Most materials are
  opaque.
• An opaque object does not allow any light to pass
  through it.

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When light strikes a new medium, the light can be
reflected, absorbed, or transmitted. When light
is transmitted, it can be refracted, polarized,
or scattered.
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• Reflection
• An image is a copy of an object formed by
  reflected (or refracted) waves of light.
• Regular reflection occurs when parallel light
  waves strike a surface and reflect all in the
  same direction.
• Diffuse reflection occurs when parallel light
  waves strike a rough, uneven surface and reflect
  in many different directions.

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Law of Reflection

• Angle of incidence (angle of incoming light rays)
  equals the angle of reflection (angle of
  reflected light rays)
• Normal line perpendicular to the surface

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• Refraction
• A light wave can refract, or bend, when it passes
  at an angle from one medium into another.
• Refraction makes underwater objects appear closer
  and larger than they really are.

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• Refraction can also sometimes cause a mirage, a
  false or distorted image.
• occur because light travels faster in hot air
  than in cooler, denser air.

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• Polarization
• Light with waves that vibrate in only one plane
  is polarized light.
• Light reflecting from a nonmetallic flat surface,
  such as a window or the surface of a lake, can
  become polarized.
• Horizontally polarized light reflects more
  strongly than the rest of the sunlight. This
  reflection produces glare. Polarized sunglasses
  have vertically polarized filters to block the
  horizontally polarized light.

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• Scattering
• In scattering, light is redirected as it passes
  through a medium.
• Most of the particles in the atmosphere are very
  small. Small particles scatter shorter-wavelength
  blue light more than light of longer wavelengths.
• Blue light is scattered in all directions more
  than other colors of light, which makes the sky
  appear blue.

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Assessment Questions

• How do polarized sunglasses reduce glare?
• by scattering light as it passes through the
  glasses
• by providing a smooth surface that light can
  reflect off
• by absorbing all light
• by blocking horizontally polarized light

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Assessment Questions

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

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Chapter 18 The Electromagnetic Spectrum and Light

• 18.4 Color

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As white light passes through a prism, shorter
wavelengths refract more than longer wavelengths,
and the colors separate.
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• Sunlight is made up of all the colors of the
  visible spectrum. A prism separates white light
  into a visible spectrum.
• When red light, with its longer wavelength,
  enters a glass prism, it slows down the least of
  all the colors.
• Red light is bent the least.
• Violet light is bent the most.

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

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The color of any object depends on what the
object is made of and on the color of light that
strikes the object.
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• An objects color is the color of light that
  reaches your eye when you look at the object.
• Sunlight contains all the colors of the visible
  spectrum.
• A red car in sunlight reflects mostly red light.
• Most of the rest of the light is absorbed at the
  surface of the paint.

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Primary colors are three specific colors that can
be combined in varying amounts to create all
possible colors.
The primary colors of light are red, green, and
blue.
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Mixing Colors of Light

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

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• If you add a primary color to the proper
  secondary color, you will get white light.
• Two colors of light that combine to form white
  light are complementary colors of light.
• A complementary color pair is a combination of
  one primary color and one secondary color.

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Light Question

• Why would a purple-people eater appear black
  under yellow light?

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The primary colors of pigments are cyan, yellow,
and magenta.
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• A pigment is a material that absorbs some colors
  of light and reflects other colors.

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Mixing Pigments

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

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Assessment Questions

• A prism separates white light into the visible
  spectrum because
• longer wavelengths are absorbed more than shorter
  wavelengths.
• shorter wavelengths refract more than longer
  wavelengths.
• shorter wavelengths reflect more than longer
  wavelengths.
• longer wavelengths experience more interference.

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Assessment Questions

• 2. Which of these colors is one of the primary
  colors of light?
• green
• magenta
• yellow
• white

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Chapter 18 The Electromagnetic Spectrum and Light

• 18.5 Sources of Light

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Common light sources include incandescent,
fluorescent, laser, neon, tungsten-halogen, and
sodium-vapor bulbs.
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• Objects that give off their own light are
  luminous. The sun is luminous, as are all light
  sources.

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• The light produced when an object gets hot enough
  to glow is incandescent.
• The filaments in incandescent light bulbs are
  made of a substance called tungsten.
• Incandescent bulbs give off most of their energy
  as heat, not light.

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• In a process called fluorescence, a material
  absorbs light at one wavelength and then emits
  light at a longer wavelength.
• A phosphor is a solid material that can emit
  light by fluorescence.
• A fluorescent bulb is a glass tube, containing
  mercury vapor, that is coated with phosphors.

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• Light in which waves have the same wavelength,
  and the crests and troughs are lined up, is
  coherent light.
• A laser is a device that generates a beam of
  coherent light.

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Neon lights emit light when electrons move
through a gas or a mixture of gases inside glass
tubing.
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• Sodium-vapor lights contain a small amount of
  solid sodium, in a mixture of neon and argon
  gases.
• The current of electrons knocks electrons in
  sodium to higher energy levels. When the
  electrons move back to lower energy levels, the
  sodium atoms emit light.

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Inside a tungsten-halogen bulb, electrons flow
through a tungsten filament. The filament gets
hot and emits light.
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Assessment Questions

• The light produced when an object becomes hot
  enough to glow is
• incandescent.
• fluorescent.
• phosphorescent.
• coherent.

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Assessment Questions

• The most efficient source of lighting rooms of a
  building is
• incandescent light.
• fluorescent light.
• sodium-vapor light.
• tungsten-halogen light.