A light beam of wavelength 500 nm is incident on a metal having work function of 1.25 eV , placed in a magnetic field of intensity B. The electrons emitted perpendicular to the magnetic field $B$, with maximum kinetic energy are bent into circular arc of radius 30 cm . The value of $B$ is $\_\_\_\_$ $\times 10^{-7} \mathrm{~T}$. Given $\mathrm{hc}=20 \times 10^{-26} \mathrm{~J}-\mathrm{m}$, mass of electron $=9 \times 10^{-31} \mathrm{~kg}$