Download Electromagnetic Wave

What are electromagnetic waves?
• Electricity can be static,
like what holds a balloon
to the wall or makes
your hair stand on
end.
• Magnetism can also be
static like a refrigerator
magnet.
• But when they change
or move together, they
make waves electromagnetic waves.
Electromagnetic Waves
• Electromagnetic
waves are formed
when an electric field
(shown as orange
arrows) couples with
a magnetic field
(shown as blue
arrows).
• The magnetic and
electric fields of an
electromagnetic wave
are perpendicular to
each other and to the
direction of the wave.
Electromagnetic Waves have different
wavelengths
• When you listen to the
radio, watch TV, or
cook dinner in a
microwave oven, you
are using
electromagnetic waves.
• Radio waves, television
waves, and
microwaves are all
types of
electromagnetic waves.
• They differ from each
other in wavelength.
Wavelength
Wavelength can be quantified by
measuring the distance between two
peaks or two troughs.
• The scientific symbol for wavelength is
a Greek letter called lambda (λ).
Wavelength
Amplitude
• Amplitude is a measurement of the
vertical distance of the wave from the
average.
• Everywhere else along the wave,
amplitudes are less than the maximum.
• Bigger ocean waves have larger wave
heights or amplitude.
Amplitude
Frequency
• Frequency refers to how many waves are
made per time interval usually described as
cycles per second.
• Usually, we use the unit Hertz to state
frequency.
• A frequency of 10 cps is noted as a
frequency of 10 Hertz. So, one cycle per
second is one Hertz.
• 1 cps = 1 Hertz
• The unit Hertz is abbreviated this way:
• 1 Hertz = 1 Hz
An animation of wave functions
with incrasing frequency
The Wave Nature of Light
GENERAL PROPERTIES OF
ELECTROMAGNETIC RADIATION
Note the trends: bluer light has shorter , higher f, and
more energy. Redder light has longer , lower f, and
less energy.
Light
Light is a form of electromagnetic
radiation, vibrating electric and
magnetic fields moving through space.
Some Basic Properties
 We call a particle of light a photon, which travels at speed c,
the speed of light, c, is a universal constant.
 c = 3 x 108 meters/sec or c = 3 x 105 kilometers/sec.
c=λxf
where λ is wavelength and f is frequency.
 Another relation between energy and frequency also given as:
E=hxf
 where E is energy of photon, h is a constant called Planck’s
constant (h=6.62 × 10−34 J∙s) and f is frequency.
 Thus, if we know the wavelength or frequency, we can compute
the energy.
Electromagnetic Spectrum
With changing
wavelength, short
wavelength to long
wavelength, we go
from:
gamma-rays > x-rays
> ultraviolet > visible
> infrared > radio
(A nanometer = nm is
10-9 m)
Wave Properties of Light
1. Interference
How interference works?
• The black wave is the
sum of the blue and
brown ones.
• When the blue one is
"in phase" with the
brown, they interfere
constructively and the
black one is larger than
either.
• When the blue one is
"out of phase", they
interfere destructively
and cancel each other
out; the black wave
vanishes.
Wave Properties
Rather than drawing the curvy lines for a wave,
sometimes we just draw a straight line for the wave
crest seen from the top (by G. Rieke)
Interference Animation
Here is how the interference animation transfers to
this version. (by G. Rieke)
Wave Properties of Light
2. Diffraction
• When light encounters
a barrier, such as a slit,
its path bends and it can
illuminate areas behind
the slit that are larger
than the width of the slit.
• Here are water waves
at a breakwater ( from J.
Alward,)
Diffraction Animation
Diffraction in action .(animation by G. Rieke)
Particle properties
1. Discrete
energies
• Photons have
specific,
discrete
energies.
2. Isolated arrival
times
Wave-Particle Duality
• If we shine photons one at a time into the box below,
we will detect them as discrete particles, each one at
a specific position against the back surface.
• However, if we collect a large number of photons,
they will distribute themselves in the "dark" areas
and avoid the "light" ones.
• The "dark" areas are where the positive wavefronts
overlap from the two slits letting each photon
through.
• Thus, they result from the combination of diffraction
and interference.
• Perhaps the most curious part is that each photon
must pass through both slits! This experiment is
called "Young's fringes" after the first scientist to do
it.
Young's Fringes
Double-slit electron diffraction
• The French physicist Louis de Broglie proposed in 1924
that electrons also have wave properties such as
wavelength and frequency.
• Later (1927) the wave nature of electrons was
experimentally established by C.J. Davisson and L.H.
Germer.
• To explain the idea, to others and themselves,
physicists often used a thought experiment, in which
Young's double-slit demonstration is repeated with a
beam of electrons instead of light.
Double-slit electron diffraction
Obeying the laws of quantum mechanics, the stream of
electrons would split in two, and the smaller streams
would interfere with each other, leaving the same kind
of light and dark fringes.
Photoelectric Effect
Light properties are related to the
temperature
• As objects get hotter, they
emit more and at shorter
wavelengths. (animation by G.
Rieke)
• The curves as shown to the
left are called blackbody
curves – they represent the
distribution over wavelength
of the energy emitted by a hot
object whose surface would
appear perfectly black if it
were cool.
• Many astronomical objects
radiate energy almost as
though they were blackbodies.