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Photo Electric Effect

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Question

Light of wavelength 2475 Å is incident on barium. Photoelectrons emitted describe a circle of radius 100 cm by a magnetic field of flux density \frac{1}{{\sqrt {17} }} \times {10^{ - 5}} Tesla. Work function of the barium is (Given  \frac{e}{m} = 1.7 \times {10^{11}})

Moderate
Solution

Radius of circular path described by a charged particle in a magnetic field is given by  r = \frac{{\sqrt {2mK} }}{{qB}};
where K = Kinetic energy of electron  \Rightarrow K = \frac{{{q^2}{B^2}{r^2}}}{{2m}} = \left( {\frac{e}{m}} \right)\frac{{e{B^2}{r^2}}}{2}
= \frac{1}{2} \times 1.7 \times {10^{11}} \times 1.6 \times {10^{ - 19}} \times {\left( {\frac{1}{{\sqrt {17} }} \times {{10}^{ - 5}}} \right)^2} \times {(1)^2}
= 8 \times {10^{ - 20}}J = 0.5\,eV
By using E = W0 + Kmax
\Rightarrow {W_0} = E - {K_{\max }} = \left( {\frac{{12375}}{{2475}}} \right)\,eV - 0.5\,eV = 4.5\,eV

 


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