An electron passing through a potential difference of $4.9 \ V$ collides with a mercury atom and transfers it to the first excited state. What is the wavelength of a photon corresponding to the transition of the mercury atom to its normal state in $\mathring{A}$?

$A$ photon collides with a stationary hydrogen atom in the ground state inelastically. The energy of the colliding photon is $10.2 \ eV$. After a time interval of the order of a microsecond,another photon collides with the same hydrogen atom inelastically with an energy of $15 \ eV$. What will be observed by the detector?

In a $He-Ne$ laser,the metastable state exists in:

The solar spectrum is an example of

When the potential energy of the second excited state of an atom is $108.8 \ eV$,the recoil momentum of the atom is ...... $kg \ m/s$.

In a Franck-Hertz experiment,an electron of energy $5.6 \ eV$ passes through mercury vapour and emerges with an energy $0.7 \ eV$. The minimum wavelength of photons emitted by mercury atoms is close to ............. $nm$.