system
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human
[Raiders of the Lost Ark] How does the light trap work?

It was the last day of Archeology at college and Professor Jones told his story of his excursion to recover the Ark of the Covenant. It was even more fun than upperclassmen have insisted for years, particularly the fantastical parts where he goes a bit off the rails (like the climax of the adventure). 

For those who haven't taken the class yet, he introduces the story by telling another, shorter story, about recovering a pagan deity bust made of solid gold from a lost temple in Peru. He describes entering an ancient stone tunnel, dealing with spiders as big as your hand, and a number of relatively advanced mechanical traps put in place by the temple builders. Some of the traps seem plausible enough, like having floor plates trigger the release of small missiles or having a balanced scale holding up the bust, but one part of the story has had me wracking my brain and I can't figure it out. 

As Prof. Jones was making his way through, he warned his companion not to cross a specific light. To prove his point, he puts his own hand in the light and out from a wall come a latticework of spears. Later, after the companion double-crossed Prof. Jones, he forgot and fell victim to the ancient trap. How might an ancient civilization have accomplished such a trap? There's some degree of sophistication in the traps, but none of them are electronic in nature, so I'm uncertain as to how interrupting light might trigger a trap.
gpt
It was really a pressure trap triggered by stepping on a floor tile.  Much like a magician using misdirection, Indy stepped down at the same time is putting his hand in the light because he suspected he would be double crossed. Upon running out the other guy avoided the light but stepped on the wrong stone.