Prepared by: Sitakshi Gupta
Electromagnetic Theory- Tutorial Sheet 2
Topics: Rotatory Polarisation & Optical Fibres
1. Calculate the required thickness of a quartz crystal whose faces are perpendicular to the optic axis in
order to produce a rotation of 43.5ᵒ for sodium light of wavelength 5893Å. The specific rotation in
quartz for this light is 21.72ᵒ/mm.
(Ans. 2.00 mm)
2. The indices of refraction of quartz for right-handed and left-handed circularly polarized light of
wavelength 7620 Å are 1.53914 and 1.53920 respectively. Calculate the rotation of the plane of
polarization of the light in degrees produced by a plate 0.5 mm thick.
(Ans. 7.1ᵒ)
3. The optical rotation produced by a material is 30ᵒ per mm at λ=5000Å and 50ᵒ per mm at λ=4000 Å.
In the visible region of the spectrum, the rotation is given by Ɵ=a+bλ-2. Calculate the constants a and
b. Also calculate the least thickness of the material which when interposed between two crossed
Nicols will produce maximum transmission for λ= 5000 Å.
(Ans. a=-5.6 deg/mm; b=8.9×108 deg mm-1 Å2, thickness=3 mm )
4. The plane of polarization of plane-polarised light is rotated through 27ᵒ in passing through a length
of 20 cm of sugar solution of 20% concentration. Calculate the specific rotation of the sugar solution.
Ans. 67.5ᵒ dm-1 (g/cm3)- 1
5. A 20 cm long tube is filled with a solution of 15 g of cane-sugar in 100 cm3 of water. Find the angle of
rotation of the plane of polarization of a beam of plane-polarised light when it passes through the
solution. Specific rotation of cane-sugar= 66.5ᵒ per decimeter/g per cm3.
(Ans. 20ᵒ)
6. 80 g of impure sugar when dissolved in a litre of water gives an optical rotation of 9.9 ᵒ when placed
in a tube of length 20 cm. If the specific rotation of sugar is 66ᵒ, find the percentage purity of sugar
sample.
(Ans. 93.75%)
7. A certain length (l1) of 5% solution causes the optical rotation of 20ᵒ. How much length of 10%
solution of the same substance will cause 35ᵒ rotation?
(Ans.
l1)
