An excited hydrogen atom releases an electromagnetic wave to return to its norma

Question

An excited hydrogen atom releases an electromagnetic wave to return to its normal state. You use your futuristic dual electric/magnetic field tester on the electromagnetic wave to find the directions of the electric field and magnetic field. Your device tells you that the electric field is pointing in the negative y direction and the magnetic field is pointing in the negative x direction. In which direction does the released electromagnetic wave travel?

+x direction ,x direction , +y direction , y direction , +z direction , z direction

It isn't -z or -x

Explanation / Answer

The direction of electromagnetic wave is described by poynting vector. Poynting vector is directly preportional to the the cross product of electric field vector and the mangetic field vector. That is

S=E X B

=(-E y)X(-B x)

=EB(yX x)

=EB(-z)

Thus the electromagnetic wave travel in negative z axis

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