Babinet compensator

to study the bireffringence of mica

1. BIREFRINGENCE OF MICA USING A BABINET COMPENSATOR

2. AIM : To determine the birefringence of mica using a Babinet compensator Isotropic material : Light travels through a material in any direction with same velocity example: glass Anisotropic or Birefringent material : Light travels through a material with two different velocities in two different directions(O- ray & E- ray) example: Quartz , Calcite Ordinary ray : The ray of light traveling along the optical axis with its electric field perpendicular it is called ordinary-ray (O-ray) Extra-ordinary ray : The ray of light with its E-field parallel to optical axis is called extra-ordinary ray (E-ray).

3. Birefringence or double refraction : Change in refractive indices of a material in two different directions

4. Babinet compe nsator: Babinet compensator shown in Figure-1 is an optical instrument designed by Jacques Babinet (1794-1872) a French physicist

5. The splitting of monochromatic light into O-ray and E-ray was first demonstrated by Babinet using an instrument called Babinet In Babinet Compensator two wedges shaped Quartz crystals cut along its two different axes as shown in the below Figure

6. On such a crystal monochromatic plane polarized light is allowed to fall. The plane polarized light gets refracted and the O-rays and E-rays that are produced interfere with each other to form fringes. These fringes can be observed. The birefringent material is placed after the polarizer on which first the plane polarized light is allowed to fall The O-rays and E-rays coming out of the material is allowed to fall on Quartz crystal of the Babinet compensator. Inside the compensator quartz crystal these rays further refract and hence the original fringes get shifted By measuring the increase in fringe shift, birefringence of the material placed between the polarizer and the compensator can be estimated

7. Birefringence pattern

8. Velocity of light in a medium is defined as V medium =c/n Where ‘c’ is velocity of light in vacuum ‘n’ is refractive index of the medium Because of double refractive index the velocities of light in two different directions are different. Hence velocity along optical axis (X-axis) V⊥= c/n o V// = c/n e c = V// n e = V⊥ n o ∆ = (n o -n e )t

9. If this path difference is integral multiple of wavelength ∆ = n λ The path difference due to second birefringent material is ∆= ( λ / β ) δβ FORMULA USED: (n o -n e ) λδβ / β t Where δβ is the fringe shift with second material β is fringe width without the second material t is the thickness of the second material λ is wavelength of the light used (no-ne) is difference in the refractive indices of the O and E rays.  

10. EXPERIMENTAL SETUP (PICTURES )

11. PROCEDURE Set up the apparatus Using the micrometer of the Babinet compensator, the fringe width is measured mica sheets is introduced between the polarizer and the compensator so there will be fringe shift Measure the fringe shift through micrometer The experiment is repeated for different fringes and each time the fringe shift is calculated and calculate the birefringence by using the formula.

12. DATA (TABULAR COLUMN) Table 1) Table 2) Mica sheet used PSR(mm) HSR (div) TR Mean thickness (mm) 1) 2) SI. NO FRINGE NO MICROMETER READING FRINGE WIDTH β (in mm) MSR CVD TOTAL READING 1) 2) 3) 4)

13. Table 3) THICKNESS (in mm) BEFORE INTRODUCING MICA AFTER INTRODUCING MICA FRING SHIFT δβ (in mm) BIREFRINGENCE (in mm) PSR HSR TR PSR HSR TR

14. FORMULA USED: (no-ne) λδβ / β t Where δβ is the fringe shift with second material β is fringe width without the second material t is the thickness of the second material λ is wavelength of the light used (no-ne) is difference in the refractive indices of the O and E rays.  

15. RESULT: BIFRINGENCE OF MICA IS FOUND TO BE =

16. THANK YOU