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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.
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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