1.
If you light something up or if it lights up , it becomes bright, usually when you shine light on it.
...a keypad that lights up when you pick up the handset...
On September 5, at the end of the festival, a massive display of fireworks will light up the sky around Broadlands.
PHRASAL VERB : V P , V P n (not pron) , also V n P
2.
If your face or your eyes light up you suddenly look very surprised or happy.
Sue’s face lit up with surprise...
You should see his eyes light up when he talks about home.
PHRASAL VERB : V P with n , V P
3.
If you light up , you make a cigarette, cigar, or pipe start burning and you start smoking it. ( INFORMAL )
He held a match while she lit up...
He took his time lighting up a cigarette.
PHRASAL VERB : V P , V P n (not pron) , also V n P