Each human eye contains roughly 120 million rod cells but only about 6 million cone cells. What does that twenty-to-one imbalance mainly explain?
- Why vision in dim light is sharp on motion and contrast but nearly colorless
- Why color vision is more accurate in dim light than in bright light
- Why the eye can regenerate a lost lens but not a damaged retina
- Why nearsightedness becomes more common with age
Why A? And why not the others?
Correct answer: A. Why vision in dim light is sharp on motion and contrast but nearly colorless
With about twenty rods for every cone, night and low-light vision leans almost entirely on the rod system, which is roughly five hundred to a thousand times more sensitive to light than cones but carries no color information at all, only brightness and motion; that is exactly why a dim room or a starlit scene looks sharp in outline yet drained of color, until enough light finally reaches the far less numerous, less sensitive cones for color vision to switch back on. The reverse claim gets the physiology backwards, since cones, not rods, carry color information, and cones need more light to function well, not less. Nothing about photoreceptor ratios relates to the eye's lens, and a damaged retina does not regenerate in humans regardless of rod-to-cone ratio. Age-related nearsightedness and its counterpart, the need for reading glasses later in life, stem from changes in the lens and its focusing muscles, an entirely different structure from the retina's photoreceptors.
Source: American Academy of Ophthalmology: Rods; Cones