A photon of energy 'E' ejects a photo electron from a metal surface whose work function is $W_0$. If this electron enters into a uniform magnetic field of induction 'B' in a direction perpendicular to the field and describes a circular path of radius 'R' , then the radius 'r' is given by , (in the usual notation):

$\begin {array} {1 1} (1)\;\sqrt {\frac{2m(E-W_0)}{eB}} & \quad (2)\;\sqrt {2m(E-W_0)eB} \\ (3)\;\frac{\sqrt {2e(E-W_0)}}{mB} & \quad (4)\;\frac{\sqrt {2m(E-W_0)}}{eB} \end {array}$

$(4)\;\frac{\sqrt {2m(E-W_0)}}{eB}$