SWITCHES

There are two rooms A and B that are totally sealed except for a door between the two rooms. There are three light switches 1, 2 and 3 in room B, only one of which operates the light in room A. There is no light at all in room B.

You are presently in room B and the door is firmly closed creating a perfect seal allowing no light to shine through whatsoever.

Your task is to discover which light switch operates the light in room A. You are allowed to open the door and enter room A only once, because the door will immediately close behind you and will not open again unless you name the correct light switch that operates the light in room A. You have only one chance to name the switch, if you do not get it right the first time you will consumed by the infectious bacteria lurking in room A!

This is not a trick question.

Question: How can you tell which switch operates the light?