From http://www.ourhouseplants.com/plants/dieffenbachia
Light
Another false belief about Dieffenbachia's is that they need very little light. This is actually wrong, light shade is acceptable but nothing darker. To maintain a healthy plant you should be giving indirect bright light. Too bright however and the leaves will scorch, too dark and you will be left with a leggy and sickly looking plant.
We have to light at least one light even during the daytime in our rooms. Rooms are small. Light spreads everywhere.
I have dumb cane. How do I protect it from the tubelight?