When we close the refrigerator door, how can we be certain the light is off?
Feel free to let loose your inner Rube Goldberg.
Rube Goldberg takes time. Modern refrigerator doors do not have a positive latch. Empty enough out to fit yourself inside, get in and close the door.
WARNING! Do NOT attempt with an old refrigerator.
More in line with your request:
Start with a photo sensor. Develop an interface with a wireless device. Stringing wires is too simple. Put a receiver outside the refrigerator. Use the outside device to light or not light a bulb the same as the one inside the refrigerator.
Put a video camera inside the fridge. Use a remote on/off device for the camera to sneak up on the bulb to make sure it isn't turning off and on by itself.
Sorry I can't come up with something more Rube Goldbergy on short notice.