Two identical photocathodes receive the light of frequencies f _ 1...

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1. A${v}_{1}^{2} - {v}_{2}^{2} = \dfrac{2h}{m}[ {f}_{1} - {f}_{2} ]$
2. B${v}_{1}^{2} + {v}_{2}^{2} = \dfrac{2h}{m}\left[{f}_{1} + {f}_{2}\right]$
3. C${v}_{1} + {v}_{2} =$\left[\dfrac{2h}{m}\left({f}_{1} + {f}_{2}\right)\right]\dfrac{1}{2}
4. D${v}_{1} - {v}_{2} = [ \dfrac{2h}{m}\left({f}_{1} - {f}_{2}\right) ]\dfrac{1}{2}$