At night, which part of the eye is least effective for vision?

The correct answer is that cones are the least effective for vision at night. This is because cones are responsible for color vision and visual acuity in bright light conditions; they function optimally in well-lit environments. During nighttime or in low-light situations, the light levels are insufficient for cones to operate effectively, leading to decreased color perception and detail recognition.

In contrast, rods are highly sensitive to low light and are better suited for night vision. They allow for detection of movement and shapes in dim conditions, although they do not provide sharp images or color differentiation. The pupil simply regulates the amount of light entering the eye and does not impact visual effectiveness directly. The fovea, which is densely populated with cones, is responsible for high acuity vision but is not effective in low-light situations. Thus, at night, the function of cones diminishes significantly, leading to their classification as the least effective part of the eye for vision in those conditions.