In a vacuum, an electromagnetic wave traveling at speed c has a relation between the magnetic field amplitude B and the electric field amplitude E which is which?

In a vacuum, an electromagnetic wave traveling at speed c has a relation between the magnetic field amplitude B and the electric field amplitude E which is which?

• A. B = E c
• B. B = E
• C. B = E / c^2
• D. B = E / c

Answer: (D)

In a vacuum, a plane electromagnetic wave has electric and magnetic fields that are perpendicular to each other and to the direction of travel, and they vary in time in phase. Maxell’s equations tie their magnitudes together: the electric field amplitude equals the speed of light times the magnetic field amplitude, so E = c B. That means B = E / c. The value c is the speed of light in vacuum, also given by 1/√(μ0 ε0). This connection reflects the vacuum impedance and why the two fields scale with c in this way. The other options would not maintain this correct ratio or match the units, so the proper relation in vacuum is B = E/c.

In a vacuum, a plane electromagnetic wave has electric and magnetic fields that are perpendicular to each other and to the direction of travel, and they vary in time in phase. Maxell’s equations tie their magnitudes together: the electric field amplitude equals the speed of light times the magnetic field amplitude, so E = c B. That means B = E / c. The value c is the speed of light in vacuum, also given by 1/√(μ0 ε0). This connection reflects the vacuum impedance and why the two fields scale with c in this way. The other options would not maintain this correct ratio or match the units, so the proper relation in vacuum is B = E/c.