ABSTRACT

In quantum theory, light is simultaneously a wave and a particle according to de Broglie’s famous formula. Light is composed of electromagnetic waves in which the electric and magnetic field amplitudes are replaced by operators. The electromagnetic spectrum represents the wavelength region of the light, as shown in Table 7.1. Light is also composed of particles called “photons” with their photon energy given by h h cu

l = [ ] [ ]

[ ] Js

m/s

m (7.1)

h h c eu l

= [Js] [m/s] [eV/ ] [m]

J (7.2)

h h c eu l

= [ ] [ ] [ ] [ ]

Js nm/s eV/J

nm

9 10 (7.3)

By substituting Plank’s constant (h = 6.626 ¥ 10-34 J.s), light speed (c = 2.998 ¥ 108 m/s) and energy (e = 6.242 ¥ 1018 eV), we will get hu

l = 1240

[ ] [ ]

nm eV (7.4)

1 eV = 8066 cm−1 wave number (cm−1) 1 Ry = 13.6 eV Rydberg (Ry) [RY] = [AU] 1 AU (1 Hartree) = 27.2 eV The speed of light is c = 2.998 × 108 m/s Table 7.1 Electromagnetic spectrum

In 1929 de Broglie received the Nobel Prize in physics for discovering the wave nature of electrons. “Electrons, like photons (particles of light waves), can act like a particle and a wave,” said de Broglie. With this discovery, he introduced a new field of study in the science of quantum physics or wave mechanics. A basic mechanical wave is shown in Fig. 7.1. Mechanical waves are disturbances through a medium (i.e., air, water, or vacuum) that usually transfer energy. They are described using several terms such as wavelength, period, frequency, amplitude, and speed of propagation.