Polarimeter

Question 1

What is an electromagnetical wave?

Answer 1

transverse wave characterized by electric + magnetic field vector perpendicular to each other and the direction of propagation

Question 2

What is non- or unpolarized light?

Answer 2

light beam whose electric field vector can be found in any plane

Question 3

What is linearly polarized light?

How can it be produced?

Answer 3

light beam whose electric field vector can be found in one particular plane

⇒ produced by polarizers

Question 4

What is circularly polarized light?

How can it be constructed?

Answer 4

light wave whose electric field vector has constant amplitude and its tip follows a helical path around the direction of propagation

can be constructed by 2 perpendicular linearly polarized waves:

• of equal amplitude
• of equal wavelength
• shifted by a quarter-wavelength

Question 5

What is a way to construct linearly polarized light other than with a polarizer?

Answer 5

adding of left- and right-handed circularly polarized light of identical amplitude

Question 6

What is the constitution of a substance?

Answer 6

the sequence of bound atoms in a compound

Question 7

Distinguish btw chiral and achiral molecules.

Answer 7

• chiral: no plane of symmetry (not superimposable on mirror image)
• achiral: plane of symmetry (superimposable)

Question 8

What are stereoisomers?

Differentiate.

Answer 8

same constitution but different spatial arrangement

• enantiomers: mirror images of each other
• diastereomers: not mirror images of each other

Question 9

What is a racemic mixture?

Answer 9

mixture of equal amounts of 2 enantiomers, hence optically inactive

Question 10

What is optical activity?

Answer 10

ability to rotate the plane of polarization of linearly polarized light

Question 11

How does the phase of a circularly polarized light shift as it passes through 2 substances with different refractive indices?

Answer 11

wavelength shortens more in substance of higher refractive index (cf. refractometer)

→ phase of resultant vector leads compared to substance of smaller refractive index

Question 12

What does Biot’s law state?

Formula.

Answer 12

since each optically active substance has its own specific rotation determined by the molecular structure, the degree of optical rotation of a solution is proportional to its concencentration and thickness

α = [α]²⁰_D * c * l

in [°*cm³/g*dm]

• α = optical rotation of solution
• [α]²⁰_D = specific rotation (20 corresponds to T in °C, D to wavelength)
• c = concentration
• l = thickness of solution

Question 13

What is a polarimeter?

What are its components?

Answer 13

measures the angle of rotation of the plane of polarization caused by optical activity
→ concentration determination

components:

• telescope with Vernier scale
• 2^nd turnable polarizer = analyser + knob
• polarimeter tube for sample solution
• fixed 1^st polarizer
• lamp housing

Question 14

How does a polarimeter work?

Answer 14

1. unpolarized monochromatic light linearly polarized by 1^st polarizer
2. passes through chiral sample in polarimeter tube → rotated by angle α
3. α measured by 2nd turnable polarizer (= analyser) on Vernier scale