A magnetic field $4 \times 10^{-2} \mathrm{~T}$ acts at right angles to a coil of area $100 \mathrm{~cm}^2$ with 50 turns. The average e.m.f. induced in the coil is 0.1 V , when it is removed from the field in time ' $t$ '. The value of ' $t$ ' is

The ratio of energies of photons produced due to transition of electron of hydrogen atom from its (i) third to $2^{\text {nd }}$ energy level and (ii) highest energy level to $3^{\text {rd }}$ level is

In Sonometer experiment, the frequency of a tuning fork used is 288 Hz . Harmonics will 'NOT' be produced at the frequency

A boy throws a ball vertically upwards from a bridge with velocity $5 \mathrm{~m} / \mathrm{s}$. It strikes water surface after 2 s . The height of the bridge is (Take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )