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  • Monochromatic radiation of wavelength 640.2 nm (1nm = 10^–9 m) from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V.

Q: 11.27  Monochromatic radiation of wavelength 640.2\hspace{1mm}nm  (1\hspace{1mm}nm=10^-^9\hspace{1mm}m)  from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54\hspace{1mm}V. The source is replaced by an iron source and its  427.2\hspace{1mm}nm line irradiates the same photo-cell. Predict the new stopping voltage. 

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The wavelength of photons emitted by the neon lamp=640.2 nm

The energy of photons emitted by the neon lamp is E given by

\\E_{1}=\frac{hc}{\lambda }\\ E_{1}=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{640.2\times 10^{-9}\times 1.6\times 10^{-19}}\\ E_{1}=1.939eV

Stopping potential is 0.54 V

Work function is therefore

\\\phi _{0}=1.939-0.54\\\phi _{0}=1.399 eV

The wavelength of photons emitted by the iron source=427.2 nm

The energy of photons emitted by the ion source is 

\\E_{2}=\frac{hc}{\lambda }\\ E_{2}=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{427.2\times 10^{-9}\times 1.6\times 10^{-19}}\\ E_{2}=2.905eV

New stopping voltage is

\\E_{2}-\phi _{0}=2.905-1.399=1.506V

 

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