A yellow light will explode (permanently destroying the light) and blind you when adjacent, but cause no other damage. The blindness will last for 10d20 turns.
The explosion of a yellow light will wake nearby sleeping monsters.
The best course of action is to put on a blindfold or towel and wait for it to explode, or to have a blindness cure ready. If it is invisible and you do not see invisible, then it won't blind you, and you get the message "Something explodes, but you get the feeling it wasn't too bright". Or alternatively zap it with a wand of cancellation and prevent it from exploding completely.
Encyclopedia entry Edit
Strange creatures formed from energy rather than matter,
lights are given to self-destructive behavior when battling