The light shield doesn’t give you a free hand for spellcasting, but since you can hold an item in the same hand that holds the light shield, you could switch your weapon to that hand to free up a hand for spellcasting. (You can’t use the weapon while it’s held in the same hand as your shield, of course.) The rules don’t state what type of action is required to switch hands on a weapon, but it seems reasonable to assume that it’s the equivalent of drawing a weapon (a move action that doesn’t provoke attacks of opportunity).