Since curtains are so rarely used, lighting has become the chief means of indicating the beginning and end of your play. Typically, "Lights up" is understood as the direction at the opening of any scene, so it's rarely written. However, a lights out direction usually does appear at the end of a scene or an act or the play. Among the common terms are "Lights fade" and "Blackout."
Collisions in Air and Space is divided into scenes rather than acts. The end of Scene 1 looks like this:
MERC I know. (He pats Alex on the back and looks out the window. Beat) There's somebody under the window. (Alex joins him at the window as they look down. Blackout.)
If it's the end of an act, it's a good idea to indicate that too. For example, in Milk and Cookies:
BRUCE For? MARGE Rufus. (Blackout and end of Act I.)
And then there's the end of the play. Here's the ending of War of the Buttons:
WALKER (Beat. Exiting) Good war. CHARLIE Yeah. You too. (Walker exits. Beat. Charlie bites into the cone, then exits as the lights dim. End of play.)
Instead of "End of play," you may wish to cling to tradition and write "Curtain."