平面電磁波 I

A plane electromagnetic wave is a wave where electric and magnetic fields are uniform over any plane perpendicular to the direction of propagation. The wavefront, a boundary plane separating field and field-free regions, moves at constant speed in the propagation direction. These waves satisfy Maxwell's equations and do not require a medium to travel through free space.

Electromagnetic waves are transverse because both electric and magnetic field vectors are perpendicular to the direction of propagation. Using Gauss's law on a rectangular Gaussian surface, the field components parallel to propagation cancel out, leaving only perpendicular components. This perpendicular orientation of oscillating fields defines the transverse nature of electromagnetic waves.

In free space, Maxwell's equations hold with zero enclosed charge and zero current. Applying Gauss's law to a Gaussian box shows that electric flux through opposite surfaces cancels, yielding zero net flux. This confirms that electromagnetic waves propagate without requiring a medium and that field components perpendicular to propagation remain non-zero while parallel components vanish.

Field components depend only on position along the propagation direction and time. The electric field is described as Ey(x, t) and Ez(x, t), while the magnetic field is Bz(x, t), with no dependence on y or z coordinates. This spatial uniformity perpendicular to propagation defines the plane wave structure and ensures fields remain constant across planes parallel to the wavefront.

Gauss's law states that net electric flux through a closed surface equals enclosed charge divided by permittivity. For a rectangular Gaussian box with zero enclosed charge, the net flux must be zero. Since field components perpendicular to propagation cancel on opposite faces, any parallel component Ex(x, t) would violate this condition, so Ex must equal zero for propagating waves.

In plane electromagnetic waves, electric and magnetic fields are perpendicular to each other and both perpendicular to the direction of propagation. The electric field oscillates in the y-direction while the magnetic field oscillates in the z-direction as the wave travels along the x-axis. This orthogonal arrangement is required by Maxwell's equations and defines the wave's transverse character.

Maxwell's equations predict electromagnetic waves in free space where no charges or currents exist. The combined electric and magnetic fields create a self-sustaining wave that propagates through empty space without needing a material medium. This prediction fundamentally changed physics by showing that light and other electromagnetic radiation travel independently of any supporting substance.