Deexcitation of electron gives us emission spectrum but in my book states it's only possible if

$\mathrm{\Delta}n\ne 0$

$\mathrm{\Delta}\ell =\pm 1$

$\mathrm{\Delta}m=0,\pm 1$

$\mathrm{\Delta}n\ne 0$

$\mathrm{\Delta}\ell =\pm 1$

$\mathrm{\Delta}m=0,\pm 1$