Continuous spectrum is not produced by which of the following?

$A$ neutron collides head-on with a stationary hydrogen atom in its ground state.

An energy of $24.6\ eV$ is required to remove one of the electrons from a neutral helium atom. The energy (in $eV$) required to remove both the electrons from a neutral helium atom is

Match the following:

$1$. Burning candle	$i$. Line spectrum
$2$. Sodium vapour	$ii$. Continuous spectrum
$3$. Bunsen flame	$iii$. Band spectrum
$4$. Dark lines in solar spectrum	$iv$. Absorption spectrum

The wavelength of a photon emitted by a stationary helium atom is $0.1 \ \mathring{A}$. What is the recoil energy of the atom in $eV$?

Which of the following elements was discovered by the study of Fraunhofer lines?