8/8/2019 Optical Spec 1 - Introduction

1/4

Physical Biochemistry Optical Spectroscopy

[Page 1]

EMR = ElectroMagnetic Radiation

Can be regarded as waves or particles wave/particle duality

Electron microscopy is not EMR as e- is being used to bombard sample acting as a particle

EMR as Waves:

Wave oscillating in the X axis = electric component

Wave oscillating in the Y axis = magnetic component

Each component is 90r to the other

Both waves propagate in the same direction

It is ok to talk about E or H as it can be assumed that the other is at 90r

Plotting the energy of a wave against time or its position in space produces an identical curve

The energy of a wave oscillates between a maximum and minimum value, with a sinusoidal form

T = time for one complete oscillation

aka periodE = Energy

= wavelength

The wavelength is the distance between the two points

of equal electrical field strength

8/8/2019 Optical Spec 1 - Introduction

2/4

Physical Biochemistry Optical Spectroscopy

[Page 2]

Frequency ( nu) [Hz]

Inverse of the period

T = period [s]

c = speed of light = 3x108m

s

Constant, EM always travel at the speed of light. = wavelength

EM as Particles:

Particles (aka wave packets) are associated as a quantum (pl. quanta) of energy

These particles are known as photos when talking about light

Photos have an associated energy

h = Plancks Constant

6.6x10-34m2kg/sE = Energy [J]

Energy is proportional to frequency: higher frequency = higher energy

Energy is inversely proportional to wavelength: longer wavelength = lower energy

1 electron Volt [eV] = 1.6x10-19 J

eV are more appropriate for the energy of photons

Type Wavelength () Frequency (v) Energy (eV)Radio ~3m ~100MHz 0.4eVMicrowave 30cm 900MHz 4eVInfrared 10m 3x1013Hz 0.1eVRed Light 650nm 5x1014Hz 2eVBlue Light 450nm 7x1014Hz 3eVUV-B 300nm 1x1015Hz 4eVX-Ray 1 3x1018Hz 13KeVWavelength / 4 gives an idea of the size of things that you can study with that wavelength

10-10m 1 10-9m 1 nm (nanometre)10-6m 1 m (micrometre)10-3m 1 mm (millimetre)10-2m 1 cm (centimetre)10-1m 1 dm (decimetre)10-0m 1 m (metre)

8/8/2019 Optical Spec 1 - Introduction

3/4

8/8/2019 Optical Spec 1 - Introduction

4/4

Physical Biochemistry Optical Spectroscopy

[Page 4]

If we excite a molecule using different wavelengths of light and look at what wavelengths are absorbed then we can

gain information about the molecular structure.

Light can be separated into different wavelengths using a prism

Emission Coloured lines correspond to the wavelengths at which light has been emitted

aka fluorescence spectroscopyEmitted light is detected at 90r to the incoming illuminating lightin order to get good separation between illuminating &

emitted light

Broad spectrum, not sharp line

Absorbance / Emission do not overlap

Similar profiles

Emission Spectroscopy:

More sensitive than absorption spectroscopy

Due to the quantummechanical nature of the photon generation process, the generation of n photons isalways associated with a minimum noise of n

Absorption and scattering may take place due to other species than the one of interestThee points are why the sensitivity of absorption spectroscopy is limited

With emission spectroscopy, single emission events can be detected

Detectors can detect single photons detection of weak phenomena is possible

More sophisticated equipment is required

Noise = n n

Increasing the number of photons (n) will reduce noise

e.g. n = 10

Noise = 10 3.333333 (33% noise)

n = 10,000

Noise = 10,000 100 (1% noise)