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Question
A photosensitive surface emits photoelectrons when red light falls on it. Will the surface emit photoelectrons when blue light is incident on it? Give reason.
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Solution
The photoelectrons can be emitted from a metal surface if the frequency of incident radiation is more than the threshold frequency.
\[\text { Photemission took place when red light is incident on the photosensitive surface} . \]
\[\text { This means that }\]
\[\text { Work function of the surface } (W) < h \nu_{red} \]
\[\text { or }, W < \frac{hc}{\lambda_{red}} . . . . . (i)\]
\[\text { When blue colour light is incident on the surface, then energy imparted to electrons }\]
\[E = \frac{hc}{\lambda_{blue}} \]
\[As \lambda_{red} > \lambda_{blue} \]
\[\text { So,} \frac{hc}{\lambda_{blue}} > \frac{hc}{\lambda_{red}} . . . . . (ii)\]
\[\text { From equation (i) and (ii)}\]
\[\frac{hc}{\lambda_{blue}} > W\]
\[\text { So, photoelectrons will be emitted when blue light is incident on the photosensitive surface }. \]
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