A light beam of wavelength $500 \mathrm{~nm}$ is incident on a metal having work function of $1.25 \mathrm{eV}$, 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 $30 \mathrm{~cm}$. The value of $B$ is _______$\times 10^{-7} \mathrm{~T}$.
Given hc $=20 \times 10^{-26} \mathrm{~J}-\mathrm{m}$, mass of electron $=9 \times 10^{-31} \mathrm{~kg}$
By photoelectric equation
$\frac{\mathrm{hc}}{\lambda}-\phi=\mathrm{k}_{\max }$
$\mathrm{k}_{\max }=\frac{1240}{500}-1.25 \approx 1.25$
$r=\frac{\sqrt{2 m k}}{e B}$
$\mathrm{B}=\frac{\sqrt{2 \mathrm{mk}}}{\mathrm{er}}$
$=125 \times 10^{-7} \mathrm{~T}$
Click here to get exam-ready with eSaral
For making your preparation journey smoother of JEE, NEET and Class 8 to 10, grab our app now.