A set of atoms in an excited state decays
(a) In general to any of the states with lower energy
(b) Into a lower state only when excited by an external electric field
(c) All together simultaneously into a lower state
(d) To emit photons only when they collide.

If work done in increasing the size of rectangular soap film with dimensions 8 cm × 3.75 cm to 10 cm × 6 cm is 2 × 10^–4 J. The surface tension of film in newton per meter is

A stationary hydrogen atom de excites from first excited state to ground state. Find recoil speed of hydrogen atom up to nearest integral value, (mass of hydrogen atom
= 1.8 × 10^–27 kg)

Electron in an hydrogen atom is excited to an energy level having energy –0.85 eV. Find the number of possible transitions it can make while deexcitation.

The ratio of kinetic energy & potential energy in 5^th excited state of Hydrogen atom is

If wavelength corresponding to 2^nd line of Lyman series is l, then the wavelength corresponding to last line of Balmer series will be: