In a plane electromagnetic wave travelling in free space, the...

Frequency of oscillation $= 2 \times {10}^{10}Hz$
Given $c = 3 \times {10}^{8}m{s}^{- 1}$,

Electric field amplitude ${E}_{0} = 48V{m}^{- 1}$

$\text{Using the formula,}c = \dfrac{{E}_{0}}{{B}_{0}}$

The amplitude of the oscillating magnetic field

$B{}_{0} = \dfrac{E{}_{0}}{c} = \dfrac{48}{3 \times 1{0}^{8}} = 1 . 6 \times 1{0}^{- 7}T$.