In a plane electromagnetic wave propagating in vacuum, the electric field E, magnetic field B, and the direction of propagation form what kind of set?

In a plane electromagnetic wave propagating in vacuum, the electric field E, magnetic field B, and the direction of propagation form what kind of set?

• A. Left-handed set
• B. E, B, and k form a collinear set
• C. A right-handed set
• D. The fields are perpendicular to propagation

Answer: (C)

In a plane electromagnetic wave in vacuum, the electric field, magnetic field, and the direction of propagation form a right-handed set. This comes from the cross product that defines the energy flow: E × B points in the direction of propagation. If you picture E and B as perpendicular to each other and both perpendicular to the propagation direction, applying the right-hand rule gives a propagation direction that matches k. For example, if E is along x and B along y, then E × B points along z, which is the direction of travel. This also matches the fact that energy travels in the propagation direction, so the triad must be right-handed. The other descriptions don’t capture this orientation: a left-handed set would imply E × B points opposite to the propagation, collinearity would mean they’re along the same line (which they aren’t), and simply being perpendicular to propagation describes E and B individually but not their collective orientation with k.

In a plane electromagnetic wave in vacuum, the electric field, magnetic field, and the direction of propagation form a right-handed set. This comes from the cross product that defines the energy flow: E × B points in the direction of propagation. If you picture E and B as perpendicular to each other and both perpendicular to the propagation direction, applying the right-hand rule gives a propagation direction that matches k. For example, if E is along x and B along y, then E × B points along z, which is the direction of travel.

This also matches the fact that energy travels in the propagation direction, so the triad must be right-handed. The other descriptions don’t capture this orientation: a left-handed set would imply E × B points opposite to the propagation, collinearity would mean they’re along the same line (which they aren’t), and simply being perpendicular to propagation describes E and B individually but not their collective orientation with k.