JEE Main & Advanced Physics Photo Electric Effect, X- Rays & Matter Waves Question Bank Critical Thinking

  • question_answer
    Let \[{{\lambda }_{\alpha }}\], \[{{\lambda }_{\beta }}\] and \[{{{\lambda }'}_{\alpha }}\] denote the wavelengths of the X-rays of the \[{{K}_{\alpha }},\,{{K}_{\beta }}\] and \[{{L}_{\alpha }}\] lines in the characteristic X-rays for a metal

    A)            \[{{\lambda }_{\alpha }}>{{{\lambda }'}_{\alpha }}>{{\lambda }_{\beta }}\]              

    B)            \[{{{\lambda }'}_{\alpha }}>{{\lambda }_{\beta }}>{{\lambda }_{\alpha }}\]

    C)            \[\frac{1}{{{\lambda }_{\beta }}}=\frac{1}{{{\lambda }_{\alpha }}}+\frac{1}{{{{{\lambda }'}}_{\alpha }}}\]   

    D)            \[\frac{1}{{{\lambda }_{\alpha }}}+\frac{1}{{{\lambda }_{\beta }}}=\frac{1}{{{{{\lambda }'}}_{\alpha }}}\]

    Correct Answer: C

    Solution :

                       According to the energy diagram of X-ray spectra                    \[\because \Delta E=\frac{hc}{\lambda }\]\[\Rightarrow \lambda \propto \frac{1}{\Delta E}\]                    (DE = Energy radiated when e? jumps from, higher energy orbit to lower energy orbit)                    \[\because {{(\Delta E)}_{{{k}_{\beta }}}}>{{(\Delta E)}_{{{k}_{\alpha }}}}>{{(\Delta E)}_{{{L}_{\alpha }}}}\]\[\therefore {{{\lambda }'}_{\alpha }}>{{\lambda }_{\alpha }}>{{\lambda }_{\beta }}\]                    Also \[{{(\Delta E)}_{{{k}_{\beta }}}}={{(\Delta E)}_{{{k}_{\alpha }}}}+{{(\Delta E)}_{{{L}_{\alpha }}}}\]                    \[\Rightarrow \frac{hc}{{{\lambda }_{\beta }}}=\frac{hc}{{{\lambda }_{\alpha }}}+\frac{hc}{{{{{\lambda }'}}_{\alpha }}}\]\[\Rightarrow \frac{1}{{{\lambda }_{\beta }}}=\frac{1}{{{\lambda }_{\alpha }}}+\frac{1}{{{{{\lambda }'}}_{\alpha }}}\]


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