# Photoelectric emission is observed from a metallic surface for frequencies $v_1$ and $v_2$ of the incident light $(v_1 > v_2)$. If the maximum values of kinetic energy of the photo electrons emitted in the two cases are in the ratio $1:n$, then the threshold frequency of the metallic surface is :

$\begin {array} {1 1} (1)\;(v_1-v_2)/(n-1) & \quad (2)\;(nv_1-v_2)/(n-1) \\ (3)\;(nv_2-v_1)/(n-1) & \quad (4)\;(v_1-v_2)/n \end {array}$

$(2)\;(nv_1-v_2)/(n-1)$