In a plane electromagnetic wave travelling in free space, the electric field component oscillates sinusoidally at a frequency of $2.0\times {10}^{10} \mathrm{Hz}$ and amplitude $48 \mathrm{V} {\mathrm{m}}^{-1}$. Then the amplitude of oscillating magnetic field is: (Speed of light in free space =$3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$)