The magnetic field \( B \) in free space is related to the electric field \( E \) in an electromagnetic wave by \( E = cB \), or \( B = \frac{E}{c} \). Given \( E = 1000 \, \text{V/m} \) and \( c = 3 \times 10^8 \, \text{m/s} \), the magnetic field is calculated as: \[ B = \frac{1000}{3 \times 10^8} = \frac{10^3}{3 \times 10^8} = \frac{1}{3} \times 10^{-5} = 0.333 \times 10^{-5} = 3.33 \times 10^{-6} \, \text{T} \] Final answer: \( 3.33 \times 10^{-6} \, \text{T} \)