Polarization of light
Basic Information:
Light
a transverse electromagnetic wave
Unpolarised Light Polarised light
oscillation of electric oscillation of electric field
field are occurring in all are occurring in a particular
possible directions direction
Unpolarised light Polarised light
Polarizer
Polarization of EM wave
Direction of E-polarization
• Light is considered polarized along Electric field
• Unpolarized light has random polarization direction
Polarizer: Polarized light
Polarizer or polaroid:
A system that allows one direction of E-vibration unperturbed
Allows other polarizations as well, but with reduced intensity
Malus’ Law
1) The 1st polarizer is used to polarize unpolarised light in a plane
2) The 2nd polarizer (analyzer) is rotated w.r.t. the 1 st polarizer by an angle
Unloparised light through polariser
The intensity of an unpolarized light across a plane polarizer also reduces
following the relation , is the intensity before polarizer
When averaged over all possible angles, the total intensity reduces by half
Polarization by reflection: Brewsters law
Brewseter’s angle
n1
n2
At this angle of incident, a
plane polarized light has
zero reflection coefficient.
So, for unpolarized
incident light, the
reflected ray will be plane
polarized and refracted ray
will be partially polarized
Look at the window!!
Superposition: elliptical and circularly polarized light
If light is composed of two plane waves of unequal amplitude by differing in
phase by 90°, then the light is said to be elliptically polarized.
( ) ( )
2 2
𝐸𝑥 𝐸𝑦
And we have 0
+ 0
=1
𝐸𝑥 𝐸𝑦
Viewed towards source
When =, we have special case of circularly polarized light
Right and left circularly polarized light
Depending on which of the x or y component is leading in phase, we
might have right or left circular polarization
Elliptic and circularly polarized light can be produced from linearly
polarized light when passed through a quarter wave plate
Polarizer crystal: Double refraction
.
z x
• Two polarised rays, namely ordinary (o-ray) and extraordinary (e-ray) is seen
when unloparised light passes through a double refracting crystal
• e-ray will rotate if the crystal is rotated
• Origin of this is associated to the underlying electron distribution of those
crystal structures
• e-ray and o-ray has same velocity along one direction along the crystal known
as optic axis
Ordinary and extraordinary light rays
O- Ray E-Ray
• Follows laws of refraction • Does not follow the Laws of refraction
• Has velocity in all • Has velocity , angle measured w.r.t.
directions in the crystal optic axis (z-axis)
• Senses a single refractive • Senses different refractive indices parallel
index for the whole crystal and perpendicular to optic axis
The different velocities of e-ray along different directions produces phase
shift. This can be used to:
1) Change polarization direction of a linearly polarized incident ray
2) Produce circularly polarized light from linearly polarized light and vice-
versa
Waveplate
A waveplate is an optical device that alters the polarization state of
a light wave travelling through it. Optic axis of a waveplate is
parallel to its surface
Quarter wave plate (QWP):
• Produces a phase difference of /2 between two polarized components
• Linear polarized beam becomes elliptically polarized across QWP
• If the plane of polarization of the incident light makes and angle of 45 o
with optic axis, the output becomes circularly polarized
Thickness of QWP:
Waveplate
Half wave plate (HWP):
• Produces a phase difference of between two polarized components
• Plane of linear polarized beam rotates by /2 across HWP when
incident in an angle of 45o with optic axis
• A right circularly polarized light will be left circularly polarized and
vice-versa across a HWP
Thickness of HWP:
Waveplate
A combination of HWP and QWP can be used to produce different
polarizations
Optical Activity
A substance is optically active if it rotates the plane of polarized light.
Dextrorotatory Substance Levorotatory Substance
• Rotation is clockwise • Rotation is counterclockwise
• Example: Glyceraldehyde • Example: D-fructose
Specific rotation [S] of a chiral substance
the rotation produced by a column of solution of length L decimeter and containing 1
gm of the active substance per cm3 of the solution at a particular temperature,
wavelength, and concentration (c)
• Length (L) measured in cm. 𝟏𝟎
• is the angle of rotation. 𝑺=
𝑳𝒄
Experimental Setup
Unpolarized Light
Polarized Light
Monochromatic Polarimeter tube
Light Source (optically active substance)
Polarizer 𝛩
Half-shade Plate
Viewer
Plane of polarization rotated
by an angle
Analyzer
