Three photons coming from excited atomic-hydrogen sample are picked up. Their energies are 12.1eV,10.2eV and 1.9eV. these photons must come from minimum of ____ atoms

$=12.1\mathrm{eV},{\mathrm{E}}_3-{\mathrm{E}}_2=1.9\mathrm{eV},{\mathrm{E}}_2-{\mathrm{E}}_1=10.2\mathrm{eV}.$

As each Hydrogen atom contain only one electron, ${\mathrm{E}}_3-{\mathrm{E}}_2$ and ${\mathrm{E}}_2-{\mathrm{E}}_1$ can take place in one atom. While  ${\mathrm{E}}_3-{\mathrm{E}}_1$take place from another atom only. Hence minimum no. of atoms required = 2.