A plane electronmagnetic wave is travelling in the positive x direction. At the instant shown the electric field at the extremely narrow dashed rectangle is in the negative z direction and its magnitude is decreasing. Which diagram correctly shows the directions and relative magnitudes of the electric field at the edges of the rectangle?

Question

Quizplus · Accepted Answer

Since the electric field is decreasing in magnitude, it means it is passing through its zero point, and hence it must be changing direction. Since the initial electric field at the rectangle is in the negative z direction, the edge closest to the origin (left edge) will experience a positive electric field (in the positive y direction), while the edge furthest from the origin (right edge) will experience a negative electric field (in the negative y direction). Option B shows this correctly.

A Plane Electronmagnetic Wave Is Travelling in the Positive X