E lectromagnetic (EM) Waves transverse waves consisting of
changing electric fields and changing magnetic fields. They differ
from mechanical waves in the way they are produced and how they
travel. They are produced by constantly changing fields. Electric
Field exerts electric forces on charged particles. Magnetic Field
produces magnetic forces produced by magnets or by changing
electric fields and vibrating charges. Electromagnetic waves are
produced when an electric charge vibrates or accelerates.
Slide 3
The figure below shows that the electric and magnetic fields
are at right angles to each other. This is a transverse wave
because the fields are also at right angles to the direction in
which the wave travels.
Slide 4
Unlike mechanical waves, electromagnetic waves do NOT need a
medium. Electromagnetic waves can travel through a vacuum, or empty
space, as well as through matter. The transfer of energy by EM
Waves traveling through matter is called electromagnetic radiation.
Why cant the electric field wave exist without the magnetic field
wave? They both produce each other!
Slide 5
Mirror Semi-silvered Mirror Michelsons Experiment- 1926
American physicist Albert Michelson measured the speed of light
more accurately than ever. He reflected and refracted light off a
mountain series of mirrors and lenses and by knowing the
differences, and by timing the light, he concluded the lights
speed. He earned the Nobel Prize in physics making him the first
American ever to get this award.
Slide 6
Light and all electromagnetic waves travel at the same speed in
a vacuum. The speed of light in a vacuum is 3.00 X 10 8 m/s.
Actually, it is 299,792,458 m/s. Even though all EM waves travel at
the same speed in a vacuum, they are not all the same. EM Waves
vary in wavelength and frequency. As we already know, the speed is
a product of its wavelength and frequency. Wave Speed = Wavelength
X Frequency A radio station broadcasts a radio wave with a
wavelength of 3.0 meters. What is the frequency of the wave?
Frequency = Wave Speed / Wavelength Frequency = 3.00 X 10 8 m/s =
1.0 X 10 8 Hz 3.0m
Slide 7
A global positioning satellite (GPS) transmits a radio wave
with a wavelength of 19cm. What is the frequency of the radio wave?
Hint: Wavelength will need to be converted to meters. Wavespeed =
Wavelength X Frequency Frequency = Wave Speed / Wavelength (3.00 X
10 8 m/s) / (0.19m) 1.6 X 10 9 Hz
Slide 8
The radio waves of an AM radio station vibrate 680,000 times
per second. What is the wavelength? Wave Speed = Wavelength X
Frequency Wave speed / Frequency = Wavelength (3.00 X 108 m/s) /
(680,000 Hz) = Wavelength (300,000,000 m/s) / (680,000/s) =
440m
Slide 9
Radio waves that vibrate 160,000,000 times per second are used
on some train lines for communications. If radio waves that vibrate
half as many times were used instead, how would the wavelength
change? At 160MHz, WL = WS / F (3.00 X 10 8 m/s) / (160,000,000Hz)
= 1.9m At 80MHz, WL = WS/F (3.00 X 10 8 m/s) / (80,000,000 Hz) =
3.8m 3.8m 1.9m = 1.9m Therefore, The wavelength would be 1.9m
longer at 80MHz than at 160MHz.
Slide 10
The fact that light casts a shadow has been used as evidence
for both the wave model of light and the particle model of light.
Evidence of the Wave Model English physicist Thomas Young in 1801,
showed that light behaves like a wave.
Slide 11
Evidence for the Particle Model The emission of electrons from
a metal caused by light striking the metal is called the
photoelectric effect. When dim blue light hits a metal such as
cesium, an electron is emitted. When a brighter blue light is
emitted, more electrons are emitted. But red light, no matter how
bright, does not cause the emission of electrons in this particular
metal. WHY???
Slide 12
In 1905, Albert Einstein proposed that light consists of
packets of energy that he named photons. Each photons energy is
proportional to the frequency of the light. The greater the
frequency, the more energy each of its photons has. Blue light has
a higher frequency than red light so the photons of blue light have
more energy than those of red light. Blue light photons have enough
energy to cause electrons to be emitted from a cesium surface.
Slide 13
Intensity The rate at which a waves energy flows through a
given unit of area. The intensity of light decreases as photons
travel farther from the source. As the light rays move farther from
the source, the lit area becomes larger, but less intense.
Slide 14
The waves of a spectrum How do you investigate something that
is invisible??? This was the problem of astronomer William Herschel
in 1800. He used a prism to separate the wavelengths present in
sunlight and placed thermometers at various places along the color
bands. He discovered that the temperature was lower at the blue
end, higher at the red end.
Slide 15
Herschel wondered if the temperature increased beyond the red
end. He concluded that the area just beyond the red are recorded an
even higher temperature showing that there must be invisible
radiation beyond the red color band. This is now called infrared
radiation.
Slide 16
The electromagnetic spectrum consists of radio waves, infrared
rays, visible light, ultraviolet rays, X-rays, and gamma rays. This
diagram shows the electromagnetic spectrum.
Slide 17
Radio waves used in radio and television technologies, as well
as in microwave ovens and radar. AM Amplitude modulation The
amplitude of the wave is varied but the frequency remains the same.
AM stations use 535 KHz 1605 KHz. FM Frequency modulation The
frequency of the wave is varied but the amplitude remains the same.
FM stations use 88MHz 108MHz.
Slide 18
The shortest wavelength radio waves are called microwaves.
These have a wavelength from about 1 meter to about 1 millimeter.
These can cook food for us. When the water or fat molecules in the
food absorb microwaves, the thermal energy of these molecules
increase. Microwaves also carry cell phone conversations, data
transfer, and high distant tv and radio transmissions.
Slide 19
Infrared rays wavelengths vary from about 1 millimeter to about
750 nanometers or millionth of a millimeter (billionth of a meter).
Infrared rays are used as a source of heat and to discover areas of
heat differences. Thermographs use infrared sensors to create
thermograms which are color-coded pictures that show variations in
temperatures. These are used to find places where a building looses
heat and problems in the path of electric current. Search and
rescue teams use infrared cameras to locate victims.
Slide 20
Visible Light Each wavelength in the visible spectrum
corresponds to a specific frequency and has a particular color.
People use visible light to see, to help keep safe, and to
communicate with one another.
Slide 21
Slide 22
Ultraviolet Rays Has higher frequency than violet light and has
applications in health, medicine, and agriculture. In moderation,
UV rays help your skin produce Vitamin D which helps the body
absorb calcium from foods which produce healthy bones and teeth.
Excessive exposure can cause skin cancer, sunburn, and wrinkles. It
can also permanently damage your eyes. UV rays are used to kill
microorganisms as well as help plants to grow.
Slide 23
X-Rays are used in medicine, industry, and transportation to
make pictures of the inside of solid objects. They have higher
frequencies than ultraviolet rays. X-Rays have high energy and can
penetrate objects that light cannot.
Slide 24
Gamma Rays Have highest frequencies and the most energy and the
greatest penetrating ability of all electromagnetic waves.
Overexposure can be deadly! Gamma rays are used in medical field to
kill cancer cells and make pictures of the brain. They are used in
industry as an inspection tool.
Slide 25
Light can be: 1.Transparent transmits light which allows most
of the light to pass through it. 2.Translucent scatters light such
as frosted glass. You can see through it but the objects are fuzzy
or lack detail. 3.Opaque absorbs or reflects all of the light that
strikes it. It does not allow any light to pass through making it
so you cant see through it.
Slide 26
When light strikes a new medium, the light can be: Reflected
Absorbed Transmitted When light is transmitted, it can be:
Refracted Polarized Scattered
Slide 27
Regular reflection occurs when parallel light waves strike a
surface and reflect all in the same direction. EX. a calm lake
acting as a mirror.
Slide 28
Diffuse reflection occurs when parallel light waves strike a
rough uneven surface and reflect in many different directions. EX.
a dog.
Slide 29
Image a copy of an object formed by reflected or refracted
waves of light. Mirage a false or distorted image. mirages occur
because light travels faster in hot air than in cooler dense air.
On a sunny day, air tends to be hotter just above the surface of a
road than higher up. Light is gradually refracted as it moves into
the layers of hotter air. This causes some of the light to curve
rather than being on a direct path to the ground.
Slide 30
Polarized light light with waves that vibrate in only one
plane. Polarized filters transmit light waves that vibrate this
way.
Slide 31
Scattering light is redirected as it passes through a medium. A
scattering of light reddens the sun at sunset and sunrise. The tiny
molecules in the Earths atmosphere can scatter sunlight causing
this.
Slide 32
The small particles in the atmosphere scatter shorter
wavelength blue light more than light of longer wavelength. By the
time the sunlight reaches your eyes, most of the blue and even some
of the green light has been scattered by the small particles. Most
of what remains are the reds and oranges. When the sun is high in
the sky, the light travels a shorter distance through the
atmosphere. It scatters blue light in all directions which explains
why the sky appears blue on a sunny day even though the air itself
has no color.
Slide 33
As white light passes through a prism, shorter wavelengths
refract more than longer wavelengths, and the colors separate. The
process in which white light separates into colors is
dispersion.
Slide 34
The color of any object depends on what the object is made of
and on the color of light that strikes the object. Sunlight
contains all of the visible colors but when you look at a yellow
car in the sunlight, the yellow paint reflects mostly yellow light.
Most of the other colors in white light are absorbed at the
surface.
Slide 35
Primary Colors three basic colors (red, green, blue) that can
be combined to produce white light. These colors can combine in
varying amounts to produce all possible colors. Secondary Colors A
combination of two primary colors making cyan, yellow, and magenta.
If you add a primary color to the proper secondary color, you get
white light. Any two colors of light that combine to form white
light are complementary colors of light. This is a secondary and
primary color that combines to form white. Blue and yellow = white
Red and cyan = white Green and magenta = white
Slide 36
Slide 37
A pigment is a material that absorbs some colors of light and
reflects other colors. Paints, inks, photographs, and dyes get
their colors from pigments. The primary colors of pigments are
cyan, yellow, and magenta. Color printers use these colors plus
black to create almost any color. The secondary colors of pigments
are red, green and blue. Any two colors of pigments that combine to
make black pigment are called complementary colors of
pigments.
Slide 38
Slide 39
Objects that give off their own source of light are luminous.
EX. Sun, lamps, headlights, flashlights, fire, etc. Common light
sources include: incandescent flourescent laser neon
tungsten-halogen sodium-vapor bulbs
Slide 40
-The light produced when an object gets hot enough to glow.
When electrons flow through the filament of an incandescent bulb,
the filament gets hot and emits light. Filament of regular light
bulbs are made of tungsten. The bulbs are filled with a mixture of
nitrogen and argon gas at a very low pressure. These gases do not
react with the filament as oxygen does so the filament doesnt burn
out as fast. Incandescent bulbs give off most of their energy as
heat.. not light!
Slide 41
During fluorescence, a material absorbs light at one wavelength
and emits light at a longer wavelength. A phosphor is a solid
material that can emit light by fluorescence. Fluorescent light
bulbs emit light by causing a phosphor to steadily emit photons. A
fluorescent bulb is a glass tube that contains mercury vapor and a
glass coated with phosphors. When current flows through the bulb,
small electrodes heat up and emit electrons. The electrons hit the
atoms of mercury and emit UV rays. The UV rays strike the phosphor
coating and emit visible light. They emit most of their energy as
light. One 18w fluorescent = one 75 w incand.
Slide 42
A laser is a device that generates a beam of coherent light.
The word laser stands for light amplification by stimulated
emission of radiation. Laser light is emitted when excited atoms of
a solid, liquid, or gas emit photons. Light in which waves have the
same wavelength, and the crests and troughs are lined up is called
coherent light. A beam of coherent light does not spread out from
its source.
Slide 43
Neon lights emit light when electrons move through a gas or a
mixture of gases inside glass tubing. Many contain gases other than
neon such as helium, argon, and krypton for the different colors
they produce.
Slide 44
Sodium-vapor lights contain small amounts of solid sodium plus
a mixture of neon and argon gases. As electric current passes
through a sodium-vapor bulb, it ionizes the gas mixture. This
mixture warms up and the heat causes the sodium to change from a
solid into a gas. The sodium atoms emit light. These are very
energy efficient and give off a very bright light but are slow to
come on. They can also alter the color of the objects below.
Slide 45
Works much like an incandescent but it has a small amount of
halogen gas such as iodine, bromine, or fluorine. These bulbs last
much longer than incandescent because the halogen gas reduces wear
on the filament. The light is made of quartz because the heat
generated can melt glass. These make colors pop and are used in
accent lighting, recessed lights, studios, and concert arenas.