The Electric Vector in Light Waves
Polarization of Light Waves
Polarized Light
Each atom produces a
wave with its own
r
orientation of E
All directions of the
electric field vector are
equally possible and
lie in a plane
perpendicular to the
direction of
propagation
A wave is said to be linearly
polarized if the resultant
electric field vibrates in the
same direction at all times at
a particular point
It may vibrate in any fixed
direction
Polarization can be obtained from
an unpolarized beam by
selective absorption
reflection
scattering
This is an unpolarized wave
Polarization by Selective Absorption
Polarization by Reflection
When an unpolarized light beam is reflected
from a surface, the reflected light is
It depends on the angle of incidence
The most common technique for polarizing light
Uses a material that transmits waves whose
electric field vectors in the plane are parallel to a
certain direction and absorbs waves whose electric
field vectors are perpendicular to that direction
E. H. Land discovered a material that polarizes light through
selective absorption
He called the material Polaroid
Liquid Crystal Displays (LCDs)
One use of polarized Light
Completely polarized
Partially polarized
Unpolarized
If the angle is 0° or 90°, the reflected beam is
unpolarized
For angles between this, there is some degree of
polarization
For one particular angle, the beam is completely polarized
The angle of incidence for which the reflected beam is
completely polarized is called the polarizing angle θp
θp is called Brewster’s Angle
n=
sinθ p
cos θ p
= tanθ p
V=0 rotates Polarization 90° making it Bright
A liquid crystal is intermediate between a
crystalline solid and a liquid
The molecules of the substance are more orderly than
those of a liquid but less than those in a pure crystalline
solid
In a display, the liquid crystal is placed between two
glass plates with electrical contacts
A voltage is applied across any segment in the display and that
segment turns on
Light passes through the polarizer on the right
and is reflected back to the observer, who sees
the segment as being bright
Magnetism-Magnetic Fields
V≠0 produces no rotation making it Dark
The light is absorbed by the polarizer on the right and
none is reflected back to the observer
Poles of a magnet are the ends where objects
are most strongly attracted
Two poles, called north and south
Like poles repel each other and unlike poles
attract each other
Similar to electric charges
Magnetic poles cannot be isolated
If a permanent magnetic is cut in half
repeatedly, you will still have a north and a
south pole
This differs from electric charges
There is some theoretical basis for
monopoles, but none have been detected
Sources of Magnetic Fields
The region of space surrounding a
moving charge includes a magnetic field
The charge will also be surrounded by
an electric field
A magnetic field surrounds a properly
magnetized magnetic material
Soft magnetic materials, such as iron, are
easily magnetized
They also tend to lose their magnetism
easily
Hard magnetic materials, such as cobalt and
nickel, are difficult to magnetize
They tend to retain their magnetism
Magnetic
Fields
r
Electric Charges in Magnetic Fields
Earth’s Magnetic Field
The Earth’s geographic north pole corresponds
to a magnetic south pole
The Earth’s geographic south pole corresponds
to a magnetic north pole
Strictly speaking, a north pole should be a
“north-seeking” pole and a south pole a
“south-seeking” pole
The red end of a compass needle which points
“North” is which end of a dipole magnet?
A. North pole
B. South pole
Moving charges feel magnetic force
perpendicular to path of
This force has a maximum value when
the charge moves perpendicularly to the
magnetic field lines
Quick Quiz
Symbolized by B
Direction is given by the direction a north pole
of a compass needle points in that location
Magnetic field lines can be used to show how
the field lines, as traced out by a compass,
would look
This force is zero when the charge moves
along the field lines
This force is zero if the charge is stationary
F = Bqv sin θ
Superconducting magnets
300000 Gauss or 30 Tesla
Earth’s magnetic field
-5 T
0.5 G or 5 x 10
Quick Quiz
How many G in a T?
D. 105
A.10-5
E. 0.5 x 10-5
B.10-4
C.104
Right Hand Rule
Place your fingers
in the
r
direction of v
Curl the fingers in the
direction
r of the magnetic
field, B
Your thumb points in the
r
direction of the force,F , on a
positive charge
If the charge is negative, the
force is opposite that determined
by the right hand rule