Getting Magnetic Field from Electric Field

Homework Statement

Hi! There is a formula in Griffiths that relates the electric field and the magnetic field:
$\mathbf{\tilde{B}}(\mathbf{r},t)=\frac{1}{c}\mathbf{\hat{k}}\times\mathbf{\tilde{E}}$

Question: Is this formula applicable for plane waves only? Can I use this for spherical waves? Thank you very much.

See above.

The Attempt at a Solution

I was told this is just for plane waves, but I want to be sure. Thank you! :)

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