Even in the dark, snakes on a plane (at least those of the pit viper and boa varieties) could keep a close watch on terrorized passengers and crew thanks to small cavities near their snouts known as pit organs. The organs are sensitive to the infrared radiation emitted by warm prey such as rats, rabbits, and Samuel L. Jackson.
(roughly the angular width of "Snakes on a Plane" costar Rachel Blanchard at three meters).
Physicists at the Technische Universität München believe the solution to the paradox could be a network of neurons in the snakes' brains –a kind of snake brain firmware - that provides image enhancement as though the snakes were wearing virtual corrective lenses. They discovered that even a crude network dramatically improves infrared imaging - which you might want to keep in mind if you ever book a flight on a snake-infested airline.
Link: Virtual Lens May Improve Heat Vision in Snakes. 2006. A. B. Sichert et al. Physical Review Letters.