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In the tenth episode of the first season of The Twilight Zone named Judgment Night which takes place during the 2nd World War a civilian ship is on its way from London to New York and is trying to escape a submarine attack from the Germans.

Whenever a character enters or leaves the dining room of the ship the lights go out as long as the door is open.

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What is the reason behind this?

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Because they have a limit switch on the door, which does this automatically when enabled. It's manually set by crew as part of dog-zebra procedure, or relay-controlled from the bridge.

The underlying purpose is to darken the ship to avoid sighting by the submarine at night. This is done in a ship's state known as Dog Zebra material condition in the US Navy, and other navies have similar procedures, as do merchant marines, for the same reason.

In movies/TV, the most well-known example of material conditions is the "Set Condition One throughout the ship" callout, used in in Battlestar Galactica 2004 and Star Trek: Deep Space Nine (on Klingon vessels), both Ronald D. Moore joints.

If you're wondering... X-ray, Yoke and Zebra are elevating levels of securing internal hatches and valves to improve watertightness. "Dog" is an amendment to "Zebra" that means "also do not emit light".

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  • That doesn't make any sense! The door is not on the same side as the windows. Windows are on the outside of the ship. I assume windows are behind the camera. The room is lit up, so light is escaping anyways. What difference does it make if light escapes into the hallway of the interior of the ship?
    – Chloe
    May 14, 2019 at 3:49
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    There's the key: "I assume the windows are behind the camera" It would be more reasonable to assume there is some kind of transparent orifice on the other side of that door from which a non-trivial amount of light could escape.
    – user45266
    May 14, 2019 at 4:40
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    Related: Some military vehicles have a similar setup where you use white light inside, but if someone opens a door the light switches to red so as to not ruin the dark vision of the person who opened the door.
    – Kapten-N
    May 14, 2019 at 9:57
  • @Chloe If there ever were any windows, they would be plated over, not least for damage control (less stuff to break).
    – Harper - Reinstate Monica
    May 14, 2019 at 14:01
  • On Starfleet ships yellow and blue alerts would probably also qualify as they do call for adjustments in the status of the ship's defensive and integrity systems (even though their forcefield technology makes having actual blast doors a waste of mass in most locations) but without bringing offensive systems online or calling the crew to battle stations.
    – Perkins
    Jun 19, 2019 at 22:56
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If you look closely you can see blackout curtains on the windows.

The door leads to an internal passageway that is open on both ends and from there allows light to leak.

Blocking out light-emitting sources to hide is important in wartime, such as in air raid blackouts.

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