Transcript 3.00 X 10 8 m/s

Electromagnetic (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.
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.
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 can’t the electric field wave
exist without the magnetic field
wave?
They both produce each other!
Michelson’s 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 light’s speed.
Mirror
Semi-silvered
Mirror
He earned the
Nobel Prize in
physics making
him the first
American ever
to get this award.
Mirror
Light and all electromagnetic waves travel at the same speed in
a vacuum. The speed of light in a vacuum is 3.00 X 108 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 108 m/s = 1.0 X 108 Hz
3.0m
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 108 m/s) / (0.19m)
1.6 X 109 Hz
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
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 108 m/s) / (160,000,000Hz) = 1.9m
At 80MHz,
WL = WS/F
(3.00 X 108 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.
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.
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???
In 1905, Albert Einstein proposed that light consists of packets of
energy that he named photons. Each photon’s 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.
Intensity – The rate at which a wave’s 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.
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.
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.
The electromagnetic spectrum consists of radio waves, infrared rays,
visible light, ultraviolet rays, X-rays, and gamma rays. This diagram
shows the electromagnetic spectrum.
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.
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.
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.
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.
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.
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.
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.
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
can’t see through it.
When light strikes a new medium, the light can be:
Reflected
Absorbed
Transmitted
When light is transmitted, it can be:
Refracted
Polarized
Scattered
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.
Diffuse reflection – occurs when parallel light waves strike a rough
uneven surface and reflect in many different directions. EX. a dog.
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.
Polarized light – light with waves that vibrate in only one plane.
Polarized filters transmit light waves that vibrate this way.
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 Earth’s atmosphere can scatter sunlight causing
this.
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.
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.
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.
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
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.
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
-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
doesn’t burn out as fast. Incandescent bulbs
give off most of their energy as heat……..
not light!
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.
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.
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.
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.
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.