Leica Polarization Microscopes 2015
Polarization microscopy
General
Polarized light can be used to analyze materials that show the Phenomenon of
Birefringence:
• Crystals (Minerals)
• Liquid Crystals
• Polymers
• Biological Samples
• Drugs
• Isotrope material with stress
birefringence (Glass)
• Liquids (flow birefringence)
Basics
1. White light
1. White light
Description of light waves:
•
•
•
Amplitude
Wave length l
Velocity of propagation
=> Brightness/Intensity
=> Colour
=> c
c
A
l
2. Polarizer
1. White light
2. Polarizer
One wavelength as example!
Description of light waves:
•
•
•
•
Amplitude
Wave length
Velocity of propagation
Situation of polarisation Ex,Ey
Ex
Ey
z
3. Sample
[Birefringence]
2. Polarizer
1. White light
3. Birefringence
3. Birefringence
The velocity of light change, when the light wave enters a material with a different
refraction index
c
__
n=
V
c = light speed (in vacuum)
V = velocity of light in the medium
Material
(isotropic)
Air
Water
Glycerin
Immersion Oil
Glass
Flint
Zircon
Diamond
Lead Sulfide
Refractive
Index (n)
1.0003
1.33
1.47
1.518
1.52
1.66
1.92
2.42
3.91
3. Birefringence
Beam path of ordinary and extraordinary waves
e
o e
o
Wave fronts of ordinary
and extraordinary
waves
c
__
no =
V1
c
__
ne =
V2
V2
V1
(modified after Raith & Raase, 2009)
3. Birefringence
OK…, well…, fine…
But were are the colors???
3. Birefringence
Sample with
birefringence
G = l/2
•
Beam is split off in two with perpendicular
vibration directions.
•
Due to different refractive indices the
waves travel with different speeds through
the material.
•
The two waves leave the sample with an
offset. (phase difference/ retardation)
nz
nx
(Raith & Raase, 2009)
Polarized light
3. Birefringence
G phase difference/
ordinary
ray
retardation
Ordinary and extraordinary ray show a
phase difference G
Phase difference is dependent on:
ne and no
extraordinary
ray
The difference of ne and no (n) is called
birefringence!
as well as from
thickness of the sample (d)
d
d x (ne – no) = G
Polarizer
This is the color!
By inserting the analyzer the phase
difference becomes visible!
4. Analyzer
3. Birefringence
2. Polarizer
1. White light
4. Interference
Dimensions of one light wave:
•
•
•
•
Amplitude
Wave length l
Velocity of propagation
Situation of polarization
=> Brightness/Intensity
=> Color
=> c
=> Ex,Ey
EY
c
A
more than one wave...?
z
Ex
l
z
4. Interference
Dimensions of one light wave:
•
•
•
•
Amplitude
Wave length l
Velocity of propagation
Situation of polarization
=> Brightness/Intensity
=> Color
=> c
=> Ex,Ey
EY
more than one wave...?
•
phase shift/ retardation
z
Ex
4. Interference
When does interference occure?
Superposition of light waves, with same velocity and
same oscillation plane (coherency).
The nature of the resulting wave depends on the phase
shift (retardation) of the interfering waves.
EY
Situation 1
constructive interference
= 0, 1l, 2l ...
Wave fronts in-phase
•
•
Addition of the Amplitudes
Higher intensity
z
Ex
4. Interference
Wave fronts dephased with different Amplitudes
Situation 2
destructive interference = lowering of the intensity
=½l
EY
z
Ex
4. Interference
Wave fronts dephased and same Amplitudes
Situation 3
EY
destructive interference = extinction of resulting wave!
=½l
z
Ex
4. Interference
constructive interference by the analyzer
Analyzer
(Raith & Raase, 2009)
4. Interference
destructive interference by the analyzer
constructive or destructive interference
(Raith & Raase, 2009)
50
The sum of constructive or destructive
interference for all wavelength result in the
0
25
Michel-Lévy-chart
G [nm]
Intensity
characteristic interference colors!
phase difference G [nm]
(Raith & Raase, 2009)
Michel-Levi-chart
G
n
Phase
difference
Birefringence n and
corresponding mineral
Thickness of sample in µm
maximum
interference colour
Application examples:
1.determination of an unknown mineral
d= must be given (25mm)
maximum
interference colour
G= can be determent with the microscope
2. Thickness determination
n= is given by the material
G= can be determent with the microscope
n =
G
d
G
d=
n
0
