Help me settle an argument (well a friendly discussion)

[ollypenrice]

There are too many variables for a clean answer. How reflective are the curtains? How reflective is the window glass? How reflective are the surfaces newly illuminated outside and how much of that reflection is directed back into the room? If someone holds mirrors up outside the window, the room will surely get lighter. If the curtains are made of glitter and the window looks onto a mat black wall, it will get darker.

It would only need a  photographic light meter to get a good idea of the reality. Most cameras have them built in...

Olly