Transcript Wave Properties - MIT Haystack Observatory

General Wave Properties
By
Leslie McGourty and Ken Rideout
All the information on waves that’s fit to print
What is a wave?
• A wave is a transfer of energy from one
point to another via a traveling disturbance
• A wave is characterized by its wavelength,
frequency, and amplitude
Transverse
• Waves that travel
perpendicular to
the direction of
motion
• Examples: Light,
-p waves
for earthquakes,
Ocean waves
Longitudinal
• Waves that travel parallel
to the direction of
motion
• Made up of compressions
and rarefactions in the
medium that they are
traveling in
• Examples: sound waves
and s waves for
earthquakes
Do You See The Difference
Between Transverse And
Longitudinal Waves?
Wavelength (λ)
• Distance from
successive crest to
crest or trough to
trough
• Measured in meters
Frequency
• Number of crests
passing by per
second
• Measured in Hertz
(Hz) defined to be
one cycle per sec
• Equal to the inverse
of the amount of time
it takes one
wavelength to pass
by
Amplitude
• Maximum displacement
of the wave
• The amplitude will have
different units depending
on the type of wave
• In a sketch of the wave, it
is the distance from the
middle of the wave to the
peak
Wave Speed
Traveling Waves move through space at a certain speed
 f v
Where,
v is the speed of the wave (m/s)
λ is the wavelength in meters (m)
f is the frequency in Hertz (cycle/s)
Matter / Quantum Waves
• Electrons and other tiny particles show wave-like
properties
• A particle moving close to the speed of light (c)
can diffract or bend around the edges of objects
• Also, particles do exhibit interference which is a
wavelike property
• Any moving matter has wave characteristics in
theory BUT the wavelength of any life-size
particle, like a golf ball, is so small that it is
negligible
• To learn about matter waves in depth go on to
the next slide; if not click
If we can sometimes consider an
electron to be a wave, what is its
wavelength?
Its wavelength depends on its
momentum
p
h

or
h

mv
where p is momentum in kg*m/s, h is Planck’s
constant = 6.63 x 10-34 J, and λ is the
wavelength in meters
What is Planck’s constant?
• Planck’s Constant is the size where quantum
mechanics becomes necessary
• Since "Planck's Constant" (‘h’= 6.63 x 10 - 34 Js)
is such a tiny number, quantum mechanics is
needed only at very small scales
• An electron also has spin that is quantized in
units of h.
• These units (Joule-sec) are units of angular
momentum
Electromagnetic Waves
• Waves of energy emitted from any accelerating charges
• Any object that is above absolute zero emits
electromagnetic waves
• The entire range of possibilities is called the
“Electromagnetic Spectrum”
• Still confused? Then click What are electromagnetic
waves?
• To learn about the wavelength of photons click to the
next slide. To move onto the EM spectrum click
Electromagnetic Waves
• Wavelength is :
c hc
 
f
E
Where,
c is the speed of light (3 x 108 m/s in a vacuum)
λ is the wavelength in meters
f is the frequency in Hertz
And
h is Planck’s constant (there it is again- do you remember its
value?)
E is the energy of a photon in Joules
What is this “photon” term you’re
throwing in there?
• A photon is a bundle (quantum) of light
• A photon has energy equal to
E  h
Recall that
h is Plank’s constant
ν is the frequency of the radiation (wave)
What does a photon do?
• Both magnetic and
electric forces involve
the exchange of
photons
• The photon has zero
rest mass, but has
momentum, can be
deflected gravity, and
can exert a force
The Electromagnetic Spectrum
• Think you know all about the
electromagnetic spectrum? Well take a
tour of the Electromagnetic Spectrum to
find out more cool information. Then, if
you’re brave enough, take the
electromagnetic quiz. Remember to run
the applet at the top of the page.
• If you still need more help review the next
7 slides. If not click
TYPES OF ELECTROMAGNETIC
WAVES
GAMMA RAYS
• Emitted from the nuclei of
atoms during radioactive
decay or during highspeed collisions with
particles.
• Ionizing
• Used in cancer treatment
and for sterilization
Sources: Cobalt 60, the
inner core of the sun
X-RAYS
• Emitted when an electron
moves from certain excited
states back down to its
ground state, or when an
electron that is moving very
quickly is suddenly stopped
• Two groups - long
wavelength (soft x-rays) and
shorter wavelength (hard xrays)
• Used for radiography (x-ray
photography) and to look at
materials in industry for
defects
• Sources: emitted by heavy
atoms after bombardment by
an electron
ULTRAVIOLET
• Above the color violet
• Three groups - UV A, UV
B, and UV C.
• “A” type: longest
wavelength; least
harmful
• UV B and UV C are
absorbed by DNA in
cells
• Used by the body to
produce vitamin D, to kill
bacteria on objects, and
for sun tanning
• Sources: Ultra hot
objects 5000°C or more
VISIBLE LIGHT
• White light:
combination of all
the colors
• Rainbow: example
of white light that
has been
separated into a
continuous
spectrum of colors
• The names of
colors are assigned
in order of their
wavelengths
• Used for
communications
(fiber optics)
• Sources: very hot
objects
Color
Wavelength interval Frequency interval
red
~ 625 to 740 nm
~ 480 to 405 THz
orange ~ 590 to 625 nm
~ 510 to 480 THz
yellow
~ 565 to 590 nm
~ 530 to 510 THz
green
~ 520 to 565 nm
~ 580 to 530 THz
cyan
~ 500 to 520 nm
~ 600 to 580 THz
blue
~ 430 to 500 nm
~ 700 to 600 THz
violet
~ 380 to 430 nm
~ 790 to 700 THz
INFRARED
• Thought of as heat but is
not always
• Far infrared energy is
heat energy.
• All objects that have
warmth radiate infrared
waves
• Easily absorbed and reradiated.
• Used in remote
controls, surveillance,
therapy of muscles
• Sources: Humans, the
sun
MICROWAVES
• 1 mm-1 dm in length
• Absorbed by water
molecules – how
microwave ovens heat
food
• Used in
telecommunications and
power transmission
• Sources: electric circuits,
many stars, microwave
ovens
RADIO WAVES
• 10 cm- 100,000+m in
length
• Only cosmic waves the
reach the surface of
the Earth
• Cause of noise
• Divided into smaller
frequency dependent
groups called bands
• Used
for communications
• Sources: transmitters
and sparks from
motors
Polarization
• Electric and magnetic fields which make
up wave have preferred direction
• Can be horizontal, vertical, circular, or
elliptical
• Most radio emission is unpolarized
• To learn more click here
Polarization
y
Electric Field
Electromagnetic Wave
Wave
Magnetic Field
x
Horizontal Polarization
Vertical Polarization
y
y
E
x
z
x
E
z
Why Do We Care About Radio
Waves?
• Gadgets- cell phones, microwaves, remote
controls, garage door openers
• Science- radio astronomy, atmospheric
research