Download The electromagnetic Spectrum

ELECTROMAGNETIC
WAVES
THE FACTS ABOUT ELECTROMAGNETIC
RADIATION
Current can be induced in a coil by a
changing magnetic field.
A changing magnetic field induces a
changing electric field at right angles to it.
A changing electric field induces a changing
magnetic field at right angles to it.
An accelerating charge has associated with
it a changing magnetic field.
Each field generates the other and can
continue to do so in any medium that can
contain electric and magnetic fields.
Therefore electromagnetic waves can be
set up by charges that oscillate backwards
and forwards.
Propagation of an electromagnetic
wave through space/air
Basic principle of an electromagnetic wave
Direction of propagation
Assume
and
represents the magnetic component
represents the electric component
generated at right angles to it
James Clark Maxwell
The genius of this man
predicted mathematically
the existence of
electromagnetic waves.
Nine years after the death of
Maxwell, Heinrich Hertz
produced and detected
electromagnetic
radio waves.
Maxwell predicts the speed of
electromagnetic waves
The permeability (μ) of a medium is a
measure of its ability to hold a magnetic field.
μ0 = the permeability of empty space
μ0 = 4 π X 10
-7
The permittivity (ε) of a medium is a
measure of its ability to hold an electric field.
ε0 = the permeability of empty space
ε 0 = (4 π X 8,988 X 10-7) -1
wave velocity = (ε0 Xμ 0 )-1/2
= 2,288 X 108 m.s-1
This is very close to the measured value
of 3,13 X 108 m.s-1
Characteristics of electromagnetic waves
• They are produced by oscillating charges.
• An oscillating electric field is at right angles
to an oscillating magnetic field.
• The two fields are in phase with each other.
• Electromagnetic waves are transverse
waves.
• Electromagnetic waves do not need a
physical medium to travel in.
• All electromagnetic waves travel at a
speed of 3 X 108 m/s (c).
• c=f.λ
• Electromagnetic waves have a particle
nature as well and the energy of the
photons E = h. f
Electromagnetic waves and energy
• c = f . λ and because c is a constant
value, when f increases λ decreases .
• E = hf , where h is Planck’s constant. The
higher the frequency of an electromagnetic
wave, the higher the energy of the
photons.
Proving that light (an electromagnetic wave)
is also a transverse wave.
Polaroid sheets have crystals of quinine
iodosulphate deposited on them.
These crystals are needle-like in shape and
have their axes parallel to each other.
The crystals transmit light vibrating in the
same plane as the crystal axis and absorb
light vibrating at right angles to this plane.
Polaroid lenses eliminating light
http://www.daviddarling.info/images/Polaroid_disks.jpg