Here are a few possibilities:

* A light source: This is the most common answer. When you turn on a light, the source (like a light bulb or a lamp) produces light that spreads into the room.

* The air itself: While air doesn't *produce* light, it acts as a medium for the light to travel through. This is why you can see light even in a seemingly empty room.

* Reflecting surfaces: Anything in the room that's reflective, like walls, furniture, or even your clothes, will reflect the light, making the room seem brighter.

Let me know if you had a specific kind of light source or effect in mind, and I'll be happy to give you a more precise answer!