1.

A radioactive element disintegrates for a time interval equal to its mean life. The fraction that has disintegrated isA. (a) `1/e`B. (b) `1-1/e`C. (c) `0.693/e`D. (d) `0.693(1-1/e)`

Answer» Correct Answer - B
Number of atoms disintegrated,
`N=N_0(1-e^(-lambdat))`
`:.` `N/N_0=1-e^(-lambdat)`
At `t=`one mean life `=1/lambda`
`N/N_0=1-e^-1=1-1/e`


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