The ratio of wavelengths of second line in Balmer series and the first line in Lyman series of hydrogen atom is

A radioactive material of half-life 2.5 hours emits radiation that is 32 times the safe maximum level. The time (in hours) after which the material can be handled safely is

If the number of uranium nuclei required per hour to produce a power of 64 kW is $7.2 \times 10^{18}$, then the energy released per fission is

According to a graph drawn between the input and output voltages of a transistor connected in common emitter configuration, the region in which transistor acts as a switch is