17.1.2 - Optical Activity

Question 1

What is the difference between unpolarised and polarised light?

Answer 1

In an unpolarised wave, oscillations may occur in any plane.
Whereas in polarised light, waves only occur in one plane

Question 2

What type of material can convert unpolarised to polarised light?

Answer 2

Polaroid

Question 3

What is polarimetry?

Answer 3

The use of a polarimeter to measure the amount of optical activity of a substance.

Question 4

What happens first in a polarimeter?

Answer 4

A monochromatic light source passes through a polarising filter.
This converts unpolarised light into vertically plane-polarised light.

Question 5

What happens in a polarimeter after vertically plane-polarised light is formed?

Answer 5

This light passes into a sample tube containing some of the substance in solution. If the substance is optically active, the plane of polarisation will be rotated so it’s no longer vertical.

Question 6

In a polarimeter, what is the name of the substance if the plane of polarisation is rotated clockwise and anticlockwise?

Answer 6

Clockwise = Dextrorotary
Anticlockwise = Laevorotatory

Question 7

What is an analyser?

Answer 7

The second polarising filter.
It’s rotated where maximum light intensity can be seen.

Question 8

What does the value of the angle of rotation of the analyser tell you about which direction is has rotated?

Answer 8

Positive value = rotated clockwise
Negative value = rotated anticlockwise

Question 9

What is significant about the plane of polarisation for enantiomers?

Answer 9

They rotate by the same angle but in different directions.
e.g One positive and one negative value

Question 10

What is a racemic mixture?

Answer 10

A mixture containing equal amounts of 2 enantiomers.
Doesn’t rotate the plane of colourised light.

Question 11

What does a polarimeter do?

Answer 11

Measures the amount of optical activity by measuring the angle of rotation caused by the substance.