Transverse wave

In a homogeneous linear medium, complex oscillations (vibrations in a material or light flows) can be described as the superposition of many simple sinusoidal waves, either transverse or longitudinal.

The vibrations of a violin string, for example, can be analyzed as the sum of many transverse waves of different frequencies, that displace the string either up or down or left to right. The ripples in a pond can be analyzed as a combination of transverse and longitudinal waves (gravity waves) that propagate together.

If the medium is linear and allows multiple independent displacement directions for the same travel direction d, we can choose two mutually perpendicular directions of polarization, and express any wave linearly polarized in any other direction as a linear combination (mixing) of those two waves.

By combining two waves with same frequency, velocity, and direction of travel, but with different phases and independent displacement directions, one obtains a circularly or elliptically polarized wave. In such a wave the particles describe circular or elliptical trajectories, instead of moving back and forth.

It may help understanding to revisit the thought experiment with a taut string mentioned above. Notice that you can also launch waves on the string by moving your hand to the right and left instead of up and down. This is an important point. There are two independent (orthogonal) directions that the waves can move. (This is true for any two directions at right angles, up and down and right and left are chosen for clarity.) Any waves launched by moving your hand in a straight line are linearly polarized waves.

But now imagine moving your hand in a circle. Your motion will launch a spiral wave on the string. You are moving your hand simultaneously both up and down and side to side. The maxima of the side to side motion occur a quarter wavelength (or a quarter of a way around the circle, that is 90 degrees or π/2 radians) from the maxima of the up and down motion. At any point along the string, the displacement of the string will describe the same circle as your hand, but delayed by the propagation speed of the wave. Notice also that you can choose to move your hand in a clockwise circle or a counter-clockwise circle. These alternate circular motions produce right and left circularly polarized waves.

To the extent your circle is imperfect, a regular motion will describe an ellipse, and produce elliptically polarized waves. At the extreme of eccentricity your ellipse will become a straight line, producing linear polarization along the major axis of the ellipse. An elliptical motion can always be decomposed into two orthogonal linear motions of unequal amplitude and 90 degrees out of phase, with circular polarization being the special case where the two linear motions have the same amplitude.

Circular polarization mechanically generated on a rubber thread, converted to linear polarization by a mechanical polarizing filter.

(Let the linear mass density of the string be μ.)

The kinetic energy of a mass element in a transverse wave is given by:

$${\displaystyle dK={\frac {1}{2}}\ dm\ v_{y}^{2}={\frac {1}{2}}\ \mu dx\ A^{2}\omega ^{2}\cos ^{2}\left({\frac {2\pi x}{\lambda }}-\omega t\right)}$$

In one wavelength, kinetic energy

$${\displaystyle K={\frac {1}{2}}\mu A^{2}\omega ^{2}\int _{0}^{\lambda }\cos ^{2}\left({\frac {2\pi x}{\lambda }}-\omega t\right)dx={\frac {1}{4}}\mu A^{2}\omega ^{2}\lambda }$$

Using Hooke's law the potential energy in mass element

$${\displaystyle dU={\frac {1}{2}}\ dm\omega ^{2}\ y^{2}={\frac {1}{2}}\ \mu dx\omega ^{2}\ A^{2}\sin ^{2}\left({\frac {2\pi x}{\lambda }}-\omega t\right)}$$

And the potential energy for one wavelength

$${\displaystyle U={\frac {1}{2}}\mu A^{2}\omega ^{2}\int _{0}^{\lambda }\sin ^{2}\left({\frac {2\pi x}{\lambda }}-\omega t\right)dx={\frac {1}{4}}\mu A^{2}\omega ^{2}\lambda }$$

So, total energy in one wavelength ${\textstyle K+U={\frac {1}{2}}\mu A^{2}\omega ^{2}\lambda }$

Therefore average power is ${\textstyle {\frac {1}{2}}\mu A^{2}\omega ^{2}v_{x}}$[3]