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Dual Nature of Radiation and Matter — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsDual Nature of Radiation and Matter5 Marks
Question
A horizontal cesium plate ($\phi$ = l.9 eV) is moved vertically downward at a constant speed u in a room full of radiation of wavelength 250 run and above. What should be the minimum value of u so that the vertically upward component of velocity is nonpositive for each photoelectron?
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Answer

Work function $=\phi,$ distance = d

The particle will move in a circle

When the stopping potential is equal to the potential due to the singly charged ion at that point.

$\text{eV}_0=\frac{\text{hc}}{\lambda}-\phi$

$\Rightarrow\text{v}_0=\Big(\frac{\text{hv}}{\lambda}-\phi\Big)\frac{1}{\text{e}}$

$\Rightarrow\frac{\text{ke}}{2\text{d}}=\Big(\frac{\text{hc}}{\lambda}-\phi\Big)\frac{1}{\text{e}}$

$\Rightarrow\frac{\text{ke}}{\text{2d}}=\frac{\text{hc}}{\lambda}-\phi$

$\Rightarrow\frac{\text{hc}}{\lambda}=\frac{\text{ke}^2}{2\text{d}}+\phi=\frac{\text{ke}^2+2\text{d}\phi}{2\text{d}}$

$\Rightarrow\lambda=\frac{\text{hc 2d}}{\text{ke}^2+2\text{d}\phi}=\frac{2\text{hcd}}{\frac{1}{4\pi\omega_0\text{e}^2}2\text{d}\phi}=\frac{8\pi\omega_0\text{hcd}}{\text{e}^2+8\pi\omega_0\text{d}\phi}$

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