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Electromagnetic Waves
Electromagnetic Waves
• An Electromagnetic wave travels like a
wave, but can interact with matter like a
particle.
• It has a “dual nature”, behaving like a
wave at times and behaving like particles
(of NO mass!) at times.
When it is
behaving like a
particle, we call
these massless
bundles of energy
photons.
• In a pure vacuum, all electromagnetic waves
travel at the speed called “c”, which we call “the
speed of light”, however the average speed
slows down a little when it travels through
substances such as water or glass as it interacts
with the molecules.
Albert Michelson is known for making
an definitive measurement of the speed of light in
the late 1800’s to early 1900’s, using a rotating
mirror apparatus between 2 California mountains
22 miles apart. His value: 299,796 km/s.
Accepted value today: 299,792 km/s
He was the first American to win the Nobel Prize
in physics.
c = 3 x 108 m/s
As is true of ALL waves,
Velocity = wavelength x frequency
So, for electromagnetic waves,
c = lf
Now, you try one….
• What is the frequency
of purple light, which
has a wavelength of
450 nm?
1 nanometer = 10-9 m
Don’t just sit there! Get out a
calculator and find the frequency.
c = lf
c = 3 x 108 m/s
l = 450 nm
(1 nm = nanometer = 10-9 m)
What is f, the frequency?
f=c÷l
f = 3 x 108 ÷ (450 x 10-9 )
f = 6.67 x 1014 Hz
How to make an electromagnetic wave:
A vibrating (accelerating!) electric charge creates a wavy
electric field that produces a wavy magnetic field that
produces an electric field that produces a magnetic
field………
…
Electromagnetic waves are transverse
waves of intertwined electrical and
magnetic fields.
“Visible Light” is only a VERY small part of the
ELECTROMAGNETIC SPECTRUM. It is the only part of the
EM spectrum that is VISIBLE to us. The entire spectrum, in
order from long wavelength to small wavelength, is
“Light”
Our color spectrum is only
a very small part of the
electromagnetic spectrum.
“Visible Light” is the part of the electromagnetic
spectrum that is in RESONANCE with the rods
and cones in our retinas.
All electromagnetic waves are the same kind
of wave.
Blue light is the same type of wave as X-rays.
Their names (microwave, infrared, radio, etc.)
are based only on a particular range of
wavelengths (or frequency).
All electromagnetic waves travel at the
“speed of light” in a vacuum.
Radio waves:
used by all TV
and Radio stations and garage door
openers, have very long wavelengths.
Frequencies:
AM radio stations broadcast in kHz.
FM stations broadcast in MHz.
TV stations broadcast in Mhz.
What is the wavelength of our public
radio station, FM 90.1?
(90.1 x 106 Hz) Using c = lf, l = ?
l = 3.3 m
Microwaves have an energy that is easily absorbed
by water molecules. This is why they are used in “microwave
ovens”.
What happens is that when the water in your TV dinner
absorbs the microwaves, it makes the water molecules
vibrate faster. The energy of the microwaves is converted
into heat.
Cell phone calls, radar and “bluetooth” are all transmitted
using microwaves. Some people are frightened that the
waves coming in and out of their cell phones are short
enough (energetic enough) to cook their brains.
(They aren’t!)
Infrared:
These waves have a
shorter wavelength than
microwaves, but their
wavelength is longer than
visible light.
IR waves are used in
remote controls for your
TV
Animals like the pit-viper
have infra-red detectors so
that they can find their prey
in the dark.
You have been warned !!
The Visible
Spectrum
Roy G Biv:
Red, Orange, Yellow, Green, Blue, Indigo,
and Violet are the colors of visible light.
Wavelength range from about
700 - 400 nm
1 Nanometer = 10-9 m
Ultraviolet:
These waves have very high energy
and very short wave lengths- shorter
than visible light.
Too much exposure to UV can cause
skin cancer- too little exposure and we
don’t produce enough Vitamin D!
UV is sometimes used to sterilize
tools, glassware, and kill bacteria on
food.
Some animals like honey bees can
see ultraviolet waves.
Even shorter wavelength:
X-Rays
have so much energy and such a
short wavelength that they can go
right through you.
They also mess with your DNA…
that’s bad
However, they cannot get through
bone as easily as they can get
through muscle.
This is because your bones contain
so much Calcium.
The very shortest, highest frequency:
Gamma rays
These are nasty ones. They have very high
energy and will even go through metals if
they are not thick layers.
Some radioactive materials produce
gamma rays (on Earth) as do super nova
explosions and pulsars (out in space).
Gamma rays and X-Rays can cause
cancer, but gamma rays can also be used
to destroy cancer cells: this is radiation
therapy.
Light and Colors
Light vs Pigments
• If light passes through
a substance, the
substance is said to
be
Light vs Pigments
• If light passes through
a substance, the
substance is said to
be transparent.
Light vs Pigments
• If light passes through
a substance, the
substance is said to
be transparent.
• If light partially passes
through a substance,
it is said to be
Light vs Pigments
• If light passes through
a substance, the
substance is said to
be transparent
• If light partially passes
through a substance,
it is said to be
translucent.
Light vs Pigments
• If light passes through a
substance, the substance
is said to be transparent.
• If light partially passes
through a substance, it is
said to be translucent.
• If light will not pass
through a substance, it is
said to be
Light vs Pigments
• If light passes through a
substance, the substance
is said to be transparent.
• If light partially passes
through a substance, it is
said to be translucent.
• If light will not pass
through a substance, it is
said to be opaque.
• An object that
produces light is
• An object that
produces light is
LUMINOUS
• An object that
produces light is
LUMINOUS
An object that
reflects light is
• An object that
produces light is
LUMINOUS
An object that
reflects light is
ILLUMINATED
Isaac Newton was the first to show that
white light consists of every color light
mixed together. He sent white light
through a prism, which produced a
rainbow of colors and then through a
second prism, where it recombined to
produce white light again. All the
colors, on atop the other, combine to
produce white light.
• The Primary colors of
VISIBLE LIGHT
• The three primary colors
of light are red, green,
and blue.
• Adding two primary colors
of light produces the
secondary colors of
light.
• Green plus blue produces
cyan.
• Blue plus red produces
magenta.
• Red plus green produces
yellow.
• Red plus green plus blue
produces white.
Two colors of light are complementary if they
produce white when added together. The
complementary pairs are:
• Green and magenta
• Red and cyan
• Blue and yellow
Pigments are chemicals that are used to produce different
colors of paint or dyes.
The primary pigments absorb ONE primary color and reflect
the other two primary colors back to our eyes.
The primary pigments are the same as the secondary colors
• Cyan pigment absorbs
red and reflects back
blue and green.
• Magenta pigment
absorbs green and
reflects blue and red.
• Yellow pigment absorbs
blue and reflects red and
green.
.
The sky is blue because the atmospheric
molecules are in resonance with blue light.
When white light comes from the sun, the blue
waves are absorbed and reemitted in all directions
by the air molecules.
So the sky will look blue and the sun will have
“lost” a little of its blue waves so that it will
look slightly yellow (red and green).
This is called “Rayleigh scattering”.
The sunrises and sunsets look yellow and orange
and red because the white light from the sun has to
pass through so much more of the atmosphere to
reach our eyes as we look towards the sun at the
horizon.
Because of that, virtually all of the blue light will
have been scattered by the air molecules, leaving
only red and green for us to see.
The oceans look blue-green because the ocean
waters (with algae and other microorganisms
and impurities) absorb red light and only reflect
back to our eyes the blue and green of the
spectrum.
Green Flash
Polarization
• If the vibrations of a wave are back and forth in one plane,
the wave is said to be polarized.
• A single vibrating electron produces a single polarized
wave.
• But a light source usually has millions of electrons
vibrating in all directions so that the waves are not
polarized.
Polarization
• A polarizer is a
type of material
that allows only
waves vibrating in
one plane (its
polarizing axis) to
pass through, so
that it produces
polarized light
waves.
Polarization
• What if light goes through two polarizing filters?
• If their polarizing axis are aligned, light will pass
through both filters. However, if the polarizing axis
are perpendicular, no light will pass through.
• Materials that rotate the
orientation of light (the plane
in which it is vibrating) as it
passes through are said to
be optically active.
• If an optically active
substance is placed
between 2 crossed
polarizing filters, some light
will pass completely through
since the polarized waves
from the first filter will have
been rotated before they
strike the second filter.