Logic Puzzle - Three Switches

[AndysDaddy]

Ok, how about this one:
A king finds his daughter “carrying on” with a commoner and orders his execution. The Princess begs her father to spare her lover and knight him so that they may marry. The King, having always been a soft touch for his only daughter, agrees to her demands on one condition. He must prove his worthiness by solving a puzzle. The man is taken down into the dungeon and shown a bank of three light switches (hey there are still Kings and Princesses, you know). Then he is shown a prison cell with a single light fixture on a table. The door to the cell is closed and the King says to the Prince-wannabe: “I will open this door again in thirty minutes, at which time you will be immediately escorted into the cell. You will then have one minute to tell me which switch operates the light. If you are right, you will be knighted and may marry my daughter. If not, you will be locked away in the cell to await your execution.

Keep in mind that this is a high tech cell door -When closed, the cell is completely sealed – no light can be seen through the door. Also, once the King opens the door again, the commoner has no opportunity to play with the switches. Finally, it is not a trick question. Exactly one of the switches operates the light in the room.