b
(b) For string, \(\frac{{{\rm{Mass}}}}{{{\rm{Length}}}}\)\( = m = \frac{{{{10}^{ - 2}}}}{{0.4}} = 2.5 \times {10^{ - 2}}kg/m\)
\(\therefore\) Velocity \(v = \sqrt {\frac{T}{m}} = \sqrt {\frac{{16}}{{2.5 \times {{10}^{ - 2}}}}} = 8m/s\)
For constructive interference between successive pulses.
\(\Delta {t_{\min }} = \frac{{2l}}{v} = \frac{{2(0.4)}}{8} = 0.1\sec \)
(After two reflections, the wave pulse is in same phase as it was produced since in one reflection it’s phase changes by \(\pi\), and If at this moment next identical pulse is produced, then constructive interference will be obtained.