Vision and Light
As adventurers explore an environment, the DM tells the players what their characters see, starting with the obvious, such as the dimensions of a corridor. If the adventurers are perceptive, they might also notice something hidden, such as a trap.
Creatures automatically see anything that is conspicuous, but they use the Perception skill to try to see something less obvious. If adventurers aren’t actively searching an area, the DM determines whether they notice hard-to-see objects or creatures by using the passive Perception of each adventurer.
Creatures normally can’t see anything without some light. Many dungeons and other indoor areas are lit, since only a few monsters are at home in utter darkness. Illumination might be provided by torches (sometimes magical ones that never stop burning), ceiling panels magically imbued with light, great oil-filled braziers or stone channels that burn continuously, or even magic globes of light that drift through the air. Natural caverns might be filled with phosphorescent fungi or lichen, extraordinary mineral veins that glimmer in the dark, streams of glowing lava, or eerie auroralike veils of magic fire undulating high above a cavern floor.
Published in Player's Handbook, page(s) 262, Rules Compendium, page(s) 166.