The title isn't the best way to describe what I want to do, probably. I'll try to explain better here. The relevant selection is below. You can see that inside the door is a bright white space, but it ends sharply below. I want to project a gradient light out below the door to hide that sharp edge and create a sort of implied floor, as if the light is shining onto the floor. This ad has what I'm talking about.
I've tried using the different light features on photoshop with little luck. If my difficulty getting across what I mean didn't tip you off, I don't have a whole lot of experience in photoshop. Is there a good way to pull this off and make it look natural?