Chemistry, Third edition

Light and Spectroscopy

Objectives

Looks at the absorption and emission of light

Explains how spectrometers work

Discusses the emission spectrum of the hydrogen atom

Outlines the uses of IR and NMR spectroscopy in analysis

Introduces the Beer–Lambert law

The interaction of light with matter may result in physical changes (such as reflec-

tion, absorption, diffraction or emission) or in chemical changes (such as dissocia-

tion or isomerization). Absorption and emission form the basis of the spectroscopic

analytical techniquesand the subject specialism is called spectroscopy. The chemi-

cal changes are no less important – the formation of smog, the depletion and forma-

tion of ozone, and photosynthesis itself, are all initiated by light.

Electromagnetic spectrum

We use the word ‘light’ to mean any radiation which is part of the electromagnetic

spectrum (Fig. 20.1), although only a small wavelength region of the spectrum

(roughly between 400 and 700 nm) is visible to the human eye.

All light travels at 3.00  108 metres per second (186 000 miles per second) in a vac-

uum. This velocity is given the symbol c.

Frequency and wavelength

Light may be thought of as being made up of waves or, alternatively, as being com-

posed of particles of pure energy called photons.

Thewavelengthof a wave is literally the ‘length of one wave’ – see Fig. 20.2. It is

symbolizedand has the units of metres.

Thefrequencyof a wave is the number of waves passing a point every second.

Frequency is symbolized (nu) and has units of Hertz (Hz). (1 Hz is one wavelength

20.1

Contents

20.1 Electromagnetic
spectrum 366

20.2 Energy levels of
atoms and
molecules 369

20.3 Spectrometers 370

20.4 Absorbance and
transmittance of a
sample 371

20.5 More about
ultraviolet and
visible spectra 373

20.6 Absorption spectra
and colour 377

20.7 Infrared
spectroscopy 379

20.8 Nuclear magnetic
resonance
spectroscopy 386

20.9 Beer–Lambert law 392

20.10Photosynthesis 395

Revision questions 396

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