PHYSICAL CHEMISTRY IN BRIEF

CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 429

12.3.2 Particle characteristics of radiation

It follows from the particle character of radiation that the energy of radiation is divided into
further indivisible amounts—quanta, which represent the energies of individual photons. The
energy of one photon depends on the radiation frequency, and is given by formula^8

E=hν , (12.44)

wherehis Planck’s constant [see12.2]. The higher the radiation frequency the higher is the
energy of individual photons. The radiation intensity [see12.1.9] is given by the product of
the number of photons passing through unit area per unit time and their energy.

Example

What is the energy of one photon of red light of the wavelength 700 nm?

Solution

E=hν=

hc

λ

=

6. 626 × 10 −^34 J s× 3 × 108 m s−^1

700 × 10 −^9 m

= 2. 84 × 10 −^19 J.

12.3.3 Spectrum

Mutual interaction between electromagnetic radiation and a substance is determined by the
quantum behaviour of photons [see12.3.2] and the quantization of the energy states of molecules
[see12.2]. The dependence of the strength of this interaction on the wavelength, frequency or
the wavenumber of the radiation is called thespectrum. E.g., the dependence of the molar
absorption coefficient [see12.1.9] on the wavelength etc. is the absorption spectrum.
Spectra may be classified based on several criteria:

• Based on the wavelength of the radiation used, spectra are classified asradiowave,
  microwave,infrared,visible light,ultravioletandX-ray.

(^8) This formula is occasionally referred to as “Planck quantum hypothesis” or “Planck law”; the latter term
is rather reserved for the historically first consequence, the Planck law of black body radiation.