Question

the sodium lamp used have a physics laboratory gives out light uniformly suppose that the lamp use 40W calculate the magnitude of Electric field

Answer 1

think there may be something missing here.  If the problem intends you to compute either the maximum or rms amplitude of the electric field, we would need to know how far from the source we are.  The electric field amplitude is related to intensity, which is power per unit area.  For an isotropic source, which emits equally in all directions, the intensity (I) decreases with distance from the source as:   I = P/(4πr2)   where   P = power of the source (given) r = distance from the source (unknown)   I will think about this more, and let you know if I come up with anything else (or perhaps another tutor will).  But if you have any information about a distance from the source, let me know, and I can help you finish the problem.  Thanks!