The ratio of amplitude of magnetic field to the amplitude of electric field of an electromagnetic wave propagating in vacuum is

For an electromagnetic wave propagating in 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 equation $E_0 = c B_0$, where $c$ is the speed of light in vacuum.

The ratio of the amplitude of the magnetic field to the amplitude of the electric field is $\dfrac{B_0}{E_0} = \dfrac{1}{c}$.

Since $c = \dfrac{1}{\sqrt{\mu_0 \epsilon_0}}$, the ratio $\dfrac{B_0}{E_0}$ is the reciprocal of the speed of light in vacuum.

Answer: reciprocal of speed of light in vacuum