Part 107 exam practice question · Operations
The retina carries two receptor types with different jobs. Cones handle colour and fine detail, need good light, and are packed into the fovea at the centre of vision. Rods are far more sensitive in low light, see no colour and resolve little detail, and populate the periphery — with none at all in the fovea itself. At night the consequence is that central vision, which is the sharpest tool available by day, becomes the weakest: looking directly at a dim object places its image where there are no rods, and the object fades. The correction is off-centre viewing, fixating about ten degrees to one side so the image falls on rod-rich retina, with small periodic shifts because rods stop responding to an unchanging stimulus. Two related losses come with the same physiology and are worth knowing: colour discrimination goes almost entirely, and depth perception degrades because the cues it depends on are weaker in the dark.
Cones concentrated in the fovea function poorly in low light and rods are absent there, so central vision is weakest at night and off-centre viewing is required to detect dim objects.
Cones provide colour and detail vision in good light and are concentrated in the fovea. Rods provide low-light sensitivity but are absent from the fovea, making off-centre viewing necessary at night, with reduced colour and depth perception.
The Centre Goes Blind at night. Your best daytime vision is your worst after dark.
Pair it with dark adaptation timing and the anti-collision lighting requirement. Night operation questions usually combine the three.
Practice 241 more Operations questions
Timed mock exams, a wrong book and flashcards — one subscription covers every exam channel.
Start studying free