Photons of energy 4.2 eV are incident on a photosensitive material of work function 1.7 eV. If the emitted photoelectrons enter normally into a uniform magnetic field of \(\sqrt{2}\times10^{-4}T\), then the largest radius of the circular path described by the photoelectrons is nearly
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For photoelectric effect problems:
\[
K_{\max}=E-\phi
\]
and for charged particles moving perpendicular to a magnetic field:
\[
r=\frac{mv}{qB}
\]
These two formulas together solve most JEE numerical questions.