I would say you can even think of it as of an adjective. Not so long ago, there was a discussion over a similar sentence: http://www.usingenglish.com/forum/as...ve-phrase.html
You probably didn't mean a particle but a preposition?
Grammatical particle - Wikipedia, the free encyclopedia
Preposition and postposition - Wikipedia, the free encyclopedia
It's not a preposition here because it doesn't introduce a prepositional phrase. The light is not on anything. It's on. "On" can be deemed to modify the subject or the verb "is". Depending on what you decide to think of it, you should choose to call it either an adjective or an adverb.
I am not not a teacher and can be wrong.
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