For an electromagnetic wave propagating in a vacuum, the relationship between the amplitude of the electric field ($E_0$) and the amplitude of the magnetic field ($B_0$) is given by the speed of light ($c$): $c = \frac{E_0}{B_0}$

The question asks for the ratio of the amplitude of the magnetic field to the amplitude of the electric field ($\frac{B_0}{E_0}$). Rearranging the formula: $\frac{B_0}{E_0} = \frac{1}{c}$ Thus, the ratio is equal to the reciprocal of the speed of light in vacuum.