It contains the experimental work performed in the laboratory.

1. Babinet compensator
Presentation by :-
Priyanka Patel
M. Sc. – I (Sem.–I)
Enll. No. – ECC2264004
Roll no. – 23004

2. Object :-
To study the Birefringence of mica
sheet by using the babinet
compensator.

3. Experimental Setup :-
 Babinet compensator.
 Mica sheet of two different thickness.
 Sodium vapour lamp (monochromatic
light source). Monochromati
c light source
Polariser Analyzer
Babinet
compensator
Image courtesy – General lab, ECC
S
N1
L L
N
C
1
2 2
M
M
Screen
Schematic Diagram :-

4. Theory & Formula Used :-
Birefringence (Δ) = λδβ
βt
Babinet compensator is made up of two wedge shaped quartz crystal cut along it’s two
different axes. When monochromatic plane polarised light is made to fall on it, the light
retracts and splits into E-Rays and O-Rays. These Rays interfere with each other to form
bright and dark Fringes.
Birefringence is the measure of ability of the material to refract light waves.
Where , δβ = Fringe shift with second material .
β = Fringe width without the second material.
t = thickness of the second material.
λ = wavelength of the light used.
Δ = difference in refractive indices of O-Rays and E-Rays.

5. Procedure :-
 Average thickness of mica sheet is
calculated.
 Fringe width is calculated by coinciding the
cross wire with two consecutive Fringes.
 Mica sheet is introduced between polariser
and the compensator.
 Fringe shift is observed as the shown beside.
 Fringe shift is calculated by coinciding the
cross wire with earlier dark fringe.
 Birefringence of mica sheet is then
calculated by using the formula mentioned
in the previous slide.

6. Observation Table :-
I). Table for thickness of mica sheet :- II). Table for Fringe width :-
Mica
sheet
used
Main
scale
Read.
(mm)
Division
on
Circular
scale
Total
reading
Mean
thickness
(t). (mm)
Sheet - 1 0.00 89 1.76
0.00 90 1.77
0.00 89 1.76
0.00 89 1.76 0.058
Sheet-2 0.00 90 1.77
0.00 91 1.78
0.00 91 1.78
0.00 90 1.77 0.023
S. No. Main
scale
read.
(mm)
Circ.
Scale
divisio
n
CSD X
LC
Total
readin
g (mm)
Fringe
width
(β)
(mm)
1. 0.5 85 0.425 0.925
2. 1.5 79 0.395 1.895 0.970
3. 2.5 82 0.410 2.910 1.015
4. 3.5 85 0.425 3.925 1.115
5. 4.5 85 0.425 4.925 1.000
6. 5.5 87 0.435 5.935 1.010
Averag
e (β)
1.022

7. (III). Table for Fringe shift :-
Sheet used MSR CSD
CSD
X
LC
Total MSR CSD
CSD
X
LC
Total Fringe shift (mm) Birefringence
0.5 85 0.425 0.925 0.0 80 0.400 0.525 0.525 0.005
1.5 94 0.470 1.970 1.0 89 0.445 1.445 0.525 0.005
0.058 2.5 96 0.480 2.980 2.0 99 0.495 2.495 0.485 0.005
3.5 91 0.455 3.955 3.0 95 0.475 3.475 0.480 0.005
4.5 97 0.485 4.985 4.0 94 0.470 4.470 0.515 0.005
0.005
0.5 90 0.450 0.950 0.0 3 0.015 0.015 0.935 0.005
1.5 89 0.445 1.945 1.0 6 0.030 1.030 0.915 0.005
0.023 2.5 92 0.460 2.960 2.0 10 0.050 2.050 0.910 0.005
3.5 95 0.475 3.975 3.0 5 0.025 3.025 0.950 0.005
4.5 88 0.440 4.940 4.0 4 0.020 4.020 0.920 0.005
0.005
(Before introducing mica sheet) (After introducing mica sheet)

8. Result :-
 Birefringence of Mica Sheet= 0.005

9. Discussion :-
 We can also calculate the ratio of axes of elliptically polarised
light.
 It may provide variable path difference, hence, preferred
over fixed wave plate where path difference is fixed.
 The whole setup should be aligned in a straight line.

10. Reference :-
 A textbook of Optics – N. Subramanhyam, Brij Lal and M. N.
Avadhanulu
 Fundamentals of Optics – Francis A. Jenkins, Harvey E.
White

11. Thank you