A plane polarized monochromatic EM wave is traveling in vacuum along z direction such that at t = t₁ it is found that the electric field is zero at a spatial point z₁. The next zero that occurs in its neighbourhood is at z₂. The frequency of the electromagnetic wave is :
1) 1.5 x 10⁸/|z₂ - z₁|                  2) 1/[t₁ + (|z₂ - z₁|/3 x 10⁸)]
3) 6 x 10⁸/|z₂ - z₁|                     4) 3 x 10⁸/|z₂ - z₁|