Electric field in plane electromagnetic wave is given by-

Ey = 5 × 10^–7 sin (0.25 × 10³x + 2.5 × 10¹¹t)

The magnetic field associated with it is

B_x = 1.66 × 10^–15 sin (0.25 × 10³x + 2.5 × 10¹¹t)

We know,

\(\frac{E_0}{B_0}\) = c

∴ B₀ = \(\frac{E_0}{c} = \frac{5×10^{-7}}{3×10^8}\) = 1.66 × 10^-15

E and B should be perpendicular to each other and also perpendicular to direction of propagation. E is along y direction and direction of propagation is along x direction hence B is along z direction.

B_z = 1.66 × 10^–15 sin (0.25 × 10³x + 10¹¹t)