Q. In a mean life of a radioactive sample
A) About $1/3$ of substance disintegrates
B) About $2/3$ of the substance disintegrates
C) About $90\%$ of the substance disintegrates
D) Almost all the substance disintegrates
Explanation
(b) By using $N = N_{0}e^{- \lambda t}$ and $t = \tau = \frac{1}{\lambda}$
Substance remains $= N = \frac{N_{0}}{e} = 0.37N_{0} \simeq \frac{N_{0}}{3}$
$\therefore$ Substance disintegrated $= N_{0} - \frac{N_{0}}{3} = \frac{2N_{0}}{3}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.