2.4: Classical Light-Waves

Consider a classical, monochromatic, linearly-polarized, plane light-wave, propagating through a vacuum in the $x$-direction. It is convenient to characterize a light-wave (which is, of course, a type of electromagnetic wave) by specifying its associated electric field. Suppose that the wave is polarized such that this electric field oscillates in the $y$-direction. (According to standard electromagnetic theory, the magnetic field oscillates in the $z$-direction, in phase with the electric field, with an amplitude which is that of the electric field divided by the velocity of light in vacuum. ) Now, the electric field can be conveniently represented in terms of a complex wavefunction:

$$\label{e2.1} \psi (x,t) = \bar{\psi}\,{\rm e}^{\,{\rm i}\,(k\,x-\omega\,t)}.$$ Here, ${\rm i} = \sqrt{-1}$, $k$ and $\omega$ are real parameters, and $\bar{\psi}$ is a complex wave amplitude. By convention, the physical electric field is the real part of the previous expression. Suppose that

$$\label{e2.2} \bar{\psi} = |\bar{\psi}|\,{\rm e}^{\,{\rm i}\,\varphi},$$ where $\varphi$ is real. It follows that the physical electric field takes the form

$$\label{e2.3} E_y(x,t) = {\rm Re}[\psi(x,t)] = |\bar{\psi}|\,\cos(k\,x-\omega\,t +\varphi),$$

where $|\bar{\psi}|$ is the amplitude of the electric oscillation, $k$ the wavenumber, $\omega$ the angular frequency, and $\varphi$ the phase angle. In addition, $\lambda=2\pi/k$ is the wavelength, and $\nu=\omega/2\pi$ the frequency (in hertz).

According to standard electromagnetic theory , the frequency and wavelength of light-waves are related according to the well-known expression

$$c = \nu\,\lambda,$$ or, equivalently,

$$\label{e2.7} \omega = k\,c,$$ where $c=3\times 10^8\,{\rm m/s}$ is the velocity of light in vacuum. Equations (2.4.3) and (2.4.5) yield

$$\label{e2.8} E_y(x,t) =|\bar{\psi}|\,\cos\left(k\,[x-(\omega/k)\,t]+\varphi\right)= |\bar{\psi}|\,\cos\left(k\,[x-c\,t]+\varphi\right).$$

Note that $E_y$ depends on $x$ and $t$ only via the combination $x-c\,t$. It follows that the wave maxima and minima satisfy

$$x - c\, t = {\rm constant}.$$ Thus, the wave maxima and minima propagate in the $x$-direction at the fixed velocity

$$\label{e2.7a} \frac{dx}{dt} = c.$$

An expression, such as Equation (2.4.5), that determines the wave angular frequency as a function of the wavenumber, is generally termed a dispersion relation. As we have already seen, and as is apparent from Equation (2.4.6), the maxima and minima of a plane-wave propagate at the characteristic velocity

$$v_p = \frac{\omega}{k},$$ which is known as the phase-velocity. Hence, the dispersion relation (2.4.5) is effectively saying that the phase-velocity of a plane light-wave, propagating through a vacuum, always takes the fixed value $c$, irrespective of its wavelength or frequency.

From standard electromagnetic theory , the energy density (i.e., the energy per unit volume) of a plane light-wave is

$$U = \frac{E_y^{\,2}}{\epsilon_0},$$ where $\epsilon_0= 8.85\times 10^{-12}\,{\rm F/m}$ is the electrical permittivity of free space. Hence, it follows from Equations (2.4.1) and (2.4.3) that

$$\label{e2.10} U \propto |\psi|^{\,2}.$$ Furthermore, a light-wave possesses linear momentum, as well as energy. This momentum is directed along the wave’s direction of propagation, and is of density

$$\label{e2.11} G = \frac{U}{c}.$$

Contributors and Attributions

• Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)

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