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Chapter 18: The Electromagnetic Spectrum and Light


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Title: Chapter 18: The Electromagnetic Spectrum and Light

Chapter 18 The Electromagnetic Spectrum and
  • 18.1 Electromagnetic Waves

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
  • 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

  • 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.

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

  • 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.

  • 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.

  • 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

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

  • 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.

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

  • 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.

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

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

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

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.

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

Chapter 18 The Electromagnetic Spectrum and Light
  • 18.2 The Electromagnetic Spectrum

The electromagnetic spectrum includes radio
waves, infrared rays, visible light, ultraviolet
rays, X-rays, and gamma rays.
  • 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.

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

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

  • 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

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

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

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

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

Chapter 18 The Electromagnetic Spectrum and Light
  • 18.3 Behavior of Light

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

  • 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.

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

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.
  • 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.

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

  • 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.

  • 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.

  • 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.

  • 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.

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

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

Chapter 18 The Electromagnetic Spectrum and Light
  • 18.4 Color

As white light passes through a prism, shorter
wavelengths refract more than longer wavelengths,
and the colors separate.
  • 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.

  • 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.

The color of any object depends on what the
object is made of and on the color of light that
strikes the object.
  • 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
  • A red car in sunlight reflects mostly red light.
  • Most of the rest of the light is absorbed at the
    surface of the paint.

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
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.

  • 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.

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

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

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.

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

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

Chapter 18 The Electromagnetic Spectrum and Light
  • 18.5 Sources of Light

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

  • 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.

  • 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.

  • 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.

Neon lights emit light when electrons move
through a gas or a mixture of gases inside glass
  • Sodium-vapor lights contain a small amount of
    solid sodium, in a mixture of neon and argon
  • 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.

Inside a tungsten-halogen bulb, electrons flow
through a tungsten filament. The filament gets
hot and emits light.
Assessment Questions
  • The light produced when an object becomes hot
    enough to glow is
  • incandescent.
  • fluorescent.
  • phosphorescent.
  • coherent.

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