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A plane polarized monochromatic EM wave is traveling in a 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. 3 × 10⁸ / |z2 − z1|
2. 6 × 10⁸ / |z2 − z1|
3. 1.5 × 10⁸ / |z2 − z1|
4. 1 / (t1 + |z2 − z1| / 3 × 10⁸)

Correct answer: 1.5 × 10⁸ / |z2 − z1|

Solution

Adjacent zeros of the field are separated by half a wavelength, so lambda = 2|z2 - z1|. The frequency f = c/lambda = (3x10^8)/(2|z2 - z1|) = 1.5x10^8 / |z2 - z1|.

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