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  • Ultraviolet light of wavelength 2271 Å from a 100 W mercury source irradiates a photo-cell made of molybdenum metal. If the stopping potential is –1.3 V, estimate the work function of the metal.

Q: 11.26 Ultraviolet light of wavelength 2271\hspace{1mm}\dot{A} from a  100\hspace{1mm}W mercury source irradiates a  photo-cell made of molybdenum metal. If the stopping potential is -1.3\hspace{1mm}V, estimate the   work function of the metal. How would the photo-cell respond to a high intensity    (\sim 10^5\hspace{1mm}Wm^2)  red light of wavelength 6382\hspace{1mm}\dot{A} produced by a He-Ne laser? 

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The energy of the incident photons is E given by

\\E=\frac{hc}{\lambda }\\ E=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{2271\times 10^{-10}\times 1.6\times 10^{-19}}\\ E=5.465\ eV

Since stopping potential is -1.3 V work function is 

\\\phi _{0}=5.465-1.3\\ \phi _{0}=4.165 eV

The energy of photons which red light consists of is ER

\\E_{R}=\frac{hc}{\lambda _{R}}\\ E_{R}=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{6382\times 10^{-10}\times 1.6\times 10^{-19}}\\ E_{R}=1.945eV

Since the energy of the photons which red light consists of have less energy than the work function, there will be no photoelectric emission when they are incident.

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