Lights on Shabbat

Question

1) May one turn off an incandescent light on Shabbat if it was accidently left on? 2) What if the light will ruin oneg Shabbat - for example, the light is on in the bedroom and will disturb sleep. 3) If one can turn of the light must he do so with a Shinui? 4) Do the answers to 1-3 above apply to florescent lights as well? 4) How about in reverse - if one accidently forgot to turn on the light before Shabbat and needs the light to learn Torah or to have a Shabbat meal which would otherwise be in darkness, may one turn on an incandescent or florescent light?