A photon of energy E ejects a photoelectron from a metal surface whose work function is 0W If electron having maximum kinetic energy 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):

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

As, $\frac{mv^2}{r}=qvB⇒\frac{mv}{qB}=r$

Also, $mv=P⇒P=\sqrt{2mk}$

$⇒r=\frac{\sqrt{2mk}}{Bq}=\frac{\sqrt{2m(E-W_0)}}{eB}$