Chemistry, Third edition

ELECTROMAGNETIC SPECTRUM

per second.) The relationship between the wavelength, frequency and the velocity of

light in a vacuum is

c=

or

c

= –



This equation shows that

a wave with a high frequency possesses a short wavelength (and vice versa)

Thequantum theoryregards light as consisting of particles of energy, called

photons. The energy E of one photon is given by the equation

E=h

wherehis a universal constant called the Planck constantwhich has the value

h= 6.626  10 –37kJs

It is sometimes more convenient to consider the energy of one mole of photons,

and to use the Planck constant per mol of photons:

hper mol = hNA= 6.626  10 –376.022 1023

 3.99 10 –13kJ s mol–1

Substituting

c

= –



forin the equation E=hgives

hc

E= —



It follows from these equations that

High-energy photons correspond to high frequency (short wavelength) light.

Low-energy photons correspond to low frequency (long wavelength) light.

367

Parameter Radiation type

Gamma X–rays Ultraviolet Visible Infrared Microwaves Radiowaves

rays () (UV) (IR)

Wavelength/m 10 –12 10 –10 10 –8 10 –4 10 –2 10

Frequency/Hz 1020 1018 1016 1012 1010 106

Photon energy/ 108 106 104 110 –2 10 –6
kJ mol–1

violet dark blue blue–green green yellow–green yellow orange red

420–450 400–420 450–490 490–530 530–545 545–580 580–630 630–720

Increasing wavelength/nm

7  1014 Hz 4  1014 Hz

Increasing frequency

300 kJ mol–1 170 kJ mol–1

Increasing photon energy

Fig. 20.1The main parts of the electromagnetic spectrum and the order of magnitudes of their
wavelengths, frequencies and photon energies. The wavelengths in the visible spectrum are
listed in nanometres (1 nm = 10–9m).

Fig. 20.2Definition of

wavelength.

Frequency and

wavelength

To receive BBC Radio 1

you require a radio tuned to

about 98 megahertz (MHz).

What wavelength is this?

Exercise 20A