Muggles' Guide to Harry Potter/Magic/Lumos

Overview
The Lumos spell creates light at the end of the caster's wand, and is therefore used for illuminating dark areas.

Extended Description
A relatively basic spell, Harry is able to use this by the end of his second year – in Harry Potter and the Chamber of Secrets, Harry uses it to illuminate his surroundings when visiting Aragog in the Forbidden Forest; but in his first year, Harry Potter and the Philosopher's Stone, he has to use a lantern, and Hermione must conjure a fire when she needs light in that book.

Analysis
The spell must be controlled by the caster's intent as well as by the word and any gestures, as it appears that not only the amount of light but the type can be controlled by the caster without any apparent difference in the casting of the spell. The light cast by the wand is described variously as a glimmer or a light, and in some cases appears as a beam, as one would get from a flashlight (UK: torch).

It is unknown how long the end of the wand remains lit; on only one occasion do we see wands consciously extinguished, by means of the Nox spell, on all other occasions they seem to conveniently go out on their own when no longer needed.