Graphite and iodine are non metals but they shine. Explain it with suitable reasons.

Because going down the group the size of the atom increases because of shielding effect etc. due to which the netineractio between the atom and the outermost electron decreases so these electrons on the surface of iodine become excited by absorbing heat energy from light so when they release this energy and come to their normal state, they emit this light so it appears that they have lustrous surface