Section B 1

Question 1

Light as a Wave -

Answer 1

Electromagnetic Radiation

James Clerk Maxwell - experiments on electricity and magnetism - Ampere, Faraday, Ohm, Hertz etc ….

Question 2

Light as a electromagnetic wave -

Answer 2

Electric (E) and Magnetic (H) fields travel together in space
they fluctuate in phase (in unison) both in time and along the propagation direction, perpendicular to the direction of travel.

Question 3

Define wavelength (lambda λ ; metres, m) ; 
frequency (ν (nu) : in s-1, or Hz) of the light wave

Answer 3

s

Question 4

The speed of travel of the light wave (in a vacuum) is a constant * :
c = ….

• but inside a substance the light speed is lowered from “c” to “v” - the ratio …. gives the refractive index n

Answer 4

λ ν = 2.998 x 10^8 m/s

Note : can approximate to 3 x 10^8 m/s (~3 x 10^10 cm/s)

the ratio c/v gives the refractive index n

Question 5

All light has ….speed c ;
λ and ν can vary

As λ increases, ν ….

Answer 5

the same

decreases (i.e., c = λν ; where c is a constant)

Question 6

We detect visible light using…. in our eyes - light transformed into electrical signals

We perceive different colours using …

Answer 6

rod and cone receptors

the cone receptors - photorhodopsin molecules

Question 7

White light - a mixture of all colours – can separate using a ….

Answer 7

prism

Question 8

“ultraviolet” (UV)

Answer 8

blackening of Ag salts

insects “see” UV light

Question 9

“infrared” (IR) -

Answer 9

use a thermometer to study

we perceive IR as “heat” : some animals “see” in the IR

Question 10

Laboratory instruments use detectors that replace our eyes

detect light in all wavelength ranges – e.g., semiconductors that …

Answer 10

transform light into electrical signals

e.g., UV-vis ; IR spectrometers

Question 11

UV-visible spectroscopy -

Answer 11

interact with electrons (transitions between quantized energy levels)

Question 12

IR spectroscopy -

Answer 12

interact with vibrations of molecules

Question 13

Radio wave region -

Answer 13

“spin flip” transitions of atomic nuclei (1H, 13C…)

Question 14

NMR spectroscopy -

Answer 14

interact with the magnetic part of light radiation : detect using radio receivers/transmitters

Question 15

The particle-wave duality is expressed by the theory of …

Answer 15

quantum mechanics (A. Einstein, M. Planck, L de Broglie, N. Bohr, W. Heisenberg ….)

Question 16

Each “particle” has an associated “pilot wave” :

Answer 16

it is termed a “photon” with a wavelength related to the “mass” (m) and particle velocity (v) by : λ = h / mv

(de Broglie relationship)

Question 17

The light energy is : E=

Answer 17

E = h ν = h c / λ : h = 6.626 x 10 -34 J s (Max Planck’s constant)

Question 18

High energy =

Answer 18

short wavelength (λ) ; high frequency (ν ) radiation

Question 19

Low energy =

Answer 19

long wavelength (λ) ; low frequency (ν ) radiation

Question 20

Use UV-visible spectroscopy to study:

Answer 20

• π-π* transitions in aromatic organic molecules
• determine chain lengths in conjugated hydrocarbons
• study C=C vs C=O unsaturated species
• determine presence of other “chromophores” : units containing -S, -P groups etc
• also organometallic, transition metal compounds : light absorption in UV-vis range due to d-d orbital transitions

Question 21

UV-visible Spectroscopy of Molecules

Answer 21

• Electron jumps occur between molecular orbitals to give rise to UV-visible absorption bands
• The positions and relative intensity of the bands gives important information on the type of bonding; also the
nature of the atoms involved : C=O ; C=C ; aromatic rings ; presence of S, P ; transition metal ions , etc …..

Question 22

• Light absorbing species are called……

Answer 22

chromophores

Question 23

IR Spectroscopy - Molecular Vibrations

Answer 23

• The atoms in molecules are not static - they vibrate around their equilibrium positions.
• The vibrational frequencies correspond to light energies (E = hν) in the infrared region of the spectrum.
• Some of the vibrations cause a change in electric dipole moment of the molecule.
• This results in a fluctuating molecular electric field associated with the vibration.
• This “resonates” with the fluctuating electric field of the light wave, causing absorption of the light
energy at each vibrational frequency.

• Some groupings of atoms always have approximately the same IR peaks in different molecules - we can use
these to work out features of the structure of an “unknown” molecule - functional group analysis :
-OH ; -NH ; -C=O ; -CH3 etc ….

Question 24

NMR Spectroscopy - Nuclear Spin Transitions

Answer 24

• Nuclei like 1H, 13C have a property called “spin”. (Isotopes - see BHP3 p14)
• The spin results in the nucleus behaving like a tiny magnet, with N-S poles.
• If the molecule is placed in an external magnetic field, the spins line up with the field direction.
• If the sample is irradiated with light (electromagnetic radiation), it can cause the spins to “flip” (note:
the interaction occurs with the fluctuating magnetic field of light).
• The spin flips have a particular energy, that depends on (a) the size of the external field; (b) the type of
nucleus; (c) the local environment (structure, bonding) of each atom.
• The spin flip energies occur in the radio wave region of the electromagnetic spectrum.
• We use radio transmitters and receivers to generate, detect the spin flip energies - an NMR spectrum.
• NMR spectroscopy provides a very detailed picture of the molecular structure, especially when
combined with other techniques like IR, UV-vis etc.

Question 25

Visible - UV

Answer 25

report λ in nm (10-9 m) : ~ 700 - 200 nm

Question 26

IR spectroscopy : Vibrations of molecules

Answer 26

λ ~ 10 - 5,000 μm

Question 27

NMR spectroscopy - nuclear spin flips - magnetic moments of atoms. Radio wave region :
ν ~ 1 - 500 MHz (106 - 109 s-1) *

Answer 27

sad

Question 28

ν ~ 1011 - 108 s-1 : microwave region :

Answer 28

(1) rotations of molecules (also sound waves in solids;
electronic transitions in semiconductors etc) - electric dipole fluctuations
(2) electron spin flip transitions - ESR spectroscopy (magnetic interactions)

Question 29

X-ray absorption -

Answer 29

electronic transitions (inner electrons)

Question 30

γ-ray spectroscopy -

Answer 30

nuclear transitions (Mo.. ssbauer)

Question 31

Raman spectroscopy -

Answer 31

illuminate with laser (visible, near-IR, UV) - observe shifts in scattered light due to molecular vibrations - complementary to IR spectroscopy

Question 32

Fluorescence -

Answer 32

electronic spectroscopy using light emitted from excited electronic states : - Imaging techniques in biology/medicine