If a hydrogen atom is excited by an energy of $8.4 \ eV$,what will be the number of spectral lines emitted?

What is the wavelength of light required to raise an electron in the $O^{7+}$ ion from $n=1$ shell to $n=2$ shell in $nm$?

For a $H$ atom,the wavelength for the highest energy transition is $91.2 \ nm$. Calculate the corresponding wavelength for $He^{+}$ in $nm$.

Why does the spectrum of an atom contain many lines even if only one electron is excited?

Values of work function $(W_0)$ for a few metals are given below:

Metal	$Li$	$Na$	$K$	$Mg$	$Cu$	$Ag$
$W_0 / eV$	$2.42$	$2.3$	$2.25$	$3.7$	$4.8$	$4.3$

The number of metals which will show photoelectric effect when light of wavelength $400 \ nm$ falls on it is $........$.
Given: $h = 6.6 \times 10^{-34} \ J \ s$,$c = 3 \times 10^8 \ m \ s^{-1}$,$e = 1.6 \times 10^{-19} \ C$

Neon gas is generally used in sign boards. If it emits strongly at $616 \, nm$,calculate $(a)$ the frequency of emission,$(b)$ distance traveled by this radiation in $30 \, s$,$(c)$ energy of a quantum,and $(d)$ number of quanta present if it produces $2 \, J$ of energy.