There's a place in the rules where it says that minor magics (e.g. Harry's light spell) shouldn't have any cost or need a roll... I'll have to look for the page number...
Edit:
Found it - Page 259 under the heading of Mundane Effects.
"Sometimes, you might want to produce an effect that doesn’t really do anything but add color to a scene. ... Any minor effect like this can be cast without a roll and will last for a scene for free."
Richard