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The illustration shows a three-dimensional coordinate system where a wave E is moving in a sinusoidal formation along the z-direction. A second figure shows a vector moving through the origin of a two-dimensional coordinate system, between (negative a sub x, a sub y) in the second quadrant and (a sub x, negative a sub y) in the fourth quadrant. The vector moves between omega t equals pi in the second quadrant and omega t equals 0 in the fourth quadrant, marking an angle, psi with the positive x-axis.