PHYSICAL CHEMISTRY IN BRIEF

CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 428

12.3 Interaction of molecules with electromagnetic

radiation

In section12.1.9we dealt with the interaction of substances with radiation (light), and ra-

diation was interpreted as a variable electromagnetic field. Radiation, however, can be also

explained as a flow of immaterial particles—photons. Given this dual character of electromag-

netic radiation, quantities of two kinds, waves and particles, are used for its description.

12.3.1 Wave characteristics of radiation

The basic characteristic of every wave motion is its frequency,ν, defined by the number of

oscillations per unit time. Another quantity is thewavelength,λ, the distance covered by the

wave during one oscillation. The wavelength depends on the speed of radiation. In vacuum it

holds

λ=

c

ν

, (12.42)

wherecis the light velocity in vacuum (c= 3× 108 m s−^1 ).

There is yet another quantity used for describing spectra, the wavenumber, ̃ν, which

represents the number of waves per unit length

ν ̃=

1

λ

=

ν

c

. (12.43)

U Main units:radiation frequency: s−^1 =Hz (one oscillation per second), wavelength: m, wavenum-
ber: m−^1.
Example
The red light frequency is approximately 4. 3 × 1014 Hz. Calculate its wavelength and wavenumber.

Solution

λ=

c

ν

=

3 × 108 m s−^1

4. 3 × 1014 s−^1

= 7× 10 −^7 m= 700nm,

̃ν=

1

λ

=

1

7 × 10 −^7 m

= 1. 43 × 106 m−^1.