Wiktionary
brighten up
vb. 1 (context intransitive said of the weather English) To become brighter 2 (context transitive English) To make cheerful.
Usage examples of "brighten up".
They were old flowers and I knew they wouldn't last for twenty-four hours, but they would brighten up our room.
People are always coming to him, suggesting that it would brighten up the dinner hour if he had singers and things, and he crushes them into little bits.
No one ate much, but everyone looked very happy, and the old room seemed to brighten up amazingly when the first romance of the family began there.
Margaret watched Jeff struggle to conceal the pain from his aching joints and a desire for a hot bath, and felt Rafaella brighten up as she rode beside her.