In an experiment on photoelectric emission from a metallic surface, wavelength of incident light is $2 \times 10^{-7}$ m and stopping potential is 2.5 V. The threshold frequency of the metal (in Hz) approximately (charge of electron $e=1.6 \times 10^{-19}C.$ Planck's constant $h= 6.6 \times 10^{34}\;J-s)$

Question

\[\begin {array} {1 1} (a)\;12 \times 10^{15} & \quad (b)\;9 \times 10^{15} \\ (c)\;9 \times 10^{14} & \quad (d)\;12 \times 10^{13} \end {array}\]