Oh, nice question. We can see an object if light from that object enters our eyes. Now the light coming from an object can either be produced by that object (example: Sun emits light ) or it can be reflected by that object into our eyes. (eg: Moon does not produce light, but we see it because it reflects sun’s light , and this light reaches our eyes). In a dark room, if no object produces any light, then nothing enters our eyes, so we cant see anything. However if there is something outside the room, which is either emitting light of its own ( eg: a bulb) or reflecting light (eg: moon or mirror) and the light rays reach our eyes then we can see it. Interestingly, just because a bulb in another room is switched on doesn’t mean we can see it, unless light rays from the bulb actually reach our eyes, we cannot see it.
