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## Chapter 18: The Electromagnetic Spectrum and Light

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Title: Chapter 18: The Electromagnetic Spectrum and Light

1
Chapter 18 The Electromagnetic Spectrum and
Light
• 18.1 Electromagnetic Waves

2
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.
3
• 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.

4
• 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.

5
• Electromagnetic waves are transverse waves
because the fields are at right angles to the
direction in which the wave travels.

6
• 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

7
• 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.

8
• 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.

9
a wave and sometimes like a stream of particles.
10
• 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.

11
• 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.

12
The intensity of light decreases as photons
travel farther from the source.
13
• 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.

14
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.

15
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?

16
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.

17
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

18
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.

19
Assessment Questions
• 4. As photons travel farther from a light source,
the intensity of light stays the same.
TrueFalse

20
Chapter 18 The Electromagnetic Spectrum and Light
• 18.2 The Electromagnetic Spectrum

21
waves, infrared rays, visible light, ultraviolet
rays, X-rays, and gamma rays.
22
• 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.

23
• Radio waves have the longest wavelengths in the
electromagnetic spectrum.
• Radio waves also have the lowest frequencies.

24
• Microwaves
• The shortest-wavelength radio waves are called
microwaves.
• Microwaves cook and reheat food. Microwaves also
carry cell phone conversations. The process works

25
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

26
• 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.

27
• 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.

28
• 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.

29
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.

30
• 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.

31
• 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.

32
• 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.

33
Assessment Questions
• Which waves have the longest wavelength?
• infrared rays
• visible light
• ultraviolet rays

34
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

35
Chapter 18 The Electromagnetic Spectrum and Light
• 18.3 Behavior of Light

36
Materials can be transparent, translucent, or
opaque.
37
• A transparent material transmits light, which
means it allows most of the light that strikes it
to pass through it.

38
• 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.

39
• 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.

40
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.
41
• 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.

42
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

43
• 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.

44
• 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.

45
• 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.

46
• 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.

47
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

48
Assessment Questions
• 2. An opaque material passes light through but
scatters the light so that objects do not look
clear. TrueFalse

49
Chapter 18 The Electromagnetic Spectrum and Light
• 18.4 Color

50
As white light passes through a prism, shorter
wavelengths refract more than longer wavelengths,
and the colors separate.
51
• 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.

52
• 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.

53
The color of any object depends on what the
object is made of and on the color of light that
strikes the object.
54
• 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.

55
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.
56
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.

57
• 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.

58
Light Question
• Why would a purple-people eater appear black
under yellow light?

59
The primary colors of pigments are cyan, yellow,
and magenta.
60
• A pigment is a material that absorbs some colors
of light and reflects other colors.

61
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.

62
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.

63
Assessment Questions
• 2. Which of these colors is one of the primary
colors of light?
• green
• magenta
• yellow
• white

64
Chapter 18 The Electromagnetic Spectrum and Light
• 18.5 Sources of Light

65
Common light sources include incandescent,
fluorescent, laser, neon, tungsten-halogen, and
sodium-vapor bulbs.
66
• Objects that give off their own light are
luminous. The sun is luminous, as are all light
sources.

67
• 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.

68
• 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.

69
• 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.

70
Neon lights emit light when electrons move
through a gas or a mixture of gases inside glass
tubing.
71
• 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.

72
Inside a tungsten-halogen bulb, electrons flow
through a tungsten filament. The filament gets
hot and emits light.
73
Assessment Questions
• The light produced when an object becomes hot
enough to glow is
• incandescent.
• fluorescent.
• phosphorescent.
• coherent.

74
Assessment Questions
• The most efficient source of lighting rooms of a
building is
• incandescent light.
• fluorescent light.
• sodium-vapor light.
• tungsten-halogen light.