I have recently put these little glow in the dark stickers around my house, to see where key points are in case the power goes out, like torches, doorways what not. Now I have found, standing there in the dark, that the little green points are bright and visible in my peripheral vision, but the minute I look at one specifically it disappears. Why is that? that my eyes see them clearly indirectly, but looking at one directly it disappears?
From this site:
"We have two types of receptors in our retina: rods and cones. The rods are responsible for achromatic vision whereas the cones are responsible for color vision. In addition to this basic difference, the rods have a much lower activation threshold than do the cones. In fact, rods are approximately 1,000 times more sensitive to light than are the cones.
Our eyes contain about six million cones and 120 million rods. The cones are concentrated at the area of greatest acuity (directly behind the lens) whereas the rods tend to occupy the periphery. At the extremes of the retina, only rods are present. This is why we can better perceive faint objects by averting our glance rather than viewing the object directly."
What zombywoof says. Practical application: In basic military training, we were taught to look indirectly at faint objects at night so we could detect them better.
Moved to General Questions from Cafe Society.
I have wondered when it is dark in my house I can’t tell if my deadbolt is locked when I look directly at it, a few feet to the side and I can tell just fine. Thanks!
Thread title edited to indicate subject.
Works at sea, too. Only by scanning past a faint object will you see it. You could be looking straight ahead towards it, but when you sweep back and forth it appears with a contrast great enough to see it.
All the same applies also to stargazing. One can see many faint stars in the sky this way, that one cannot see while looking straight towards them.
I encountered this phenom also when looking at a naked-eye-visible comet that passed by a few years ago. (Forgot the name; one of the two in the mid-1990’s.) When it first became visible, it was quite low on the horizon in the evening sky, just barely visible among some tree-tops. It appeared as a very faint and tiny fuzzball, which I could only see by not looking at it.
I also use this trick for looking for stars. And I have specifically noticed it with glow in the dark stickers, so perhaps there’s something about glow in the dark stickers that makes it more pronounced. I’d guess it’s that they gradually decrease in brightness so there will always be a point where you can see them with peripheral vision but not when looking directly at them.
There are, in fact, no rods at all in the central part of the retina, the fovea, where most of the cones are, and which provides our ability to see color and fine detail (because the cones there are packed very densely. The numbers of L and M cones (roughly, red sensitive and green sensitive, respectively) falls off rapidly away from the fovea, and there are none in the periphery, which is mostly rods (but distributed much more sparsely than the cones in the fovea), plus a few S-cones (the ones most sensitive to blue light), even more sparsely distributed.
As you rightly say, in dim light the cones are not functional, we are relying entirely on rods, and this is why you can’t see things at all when you look straight on at them. However, I wanted to point out that in brighter conditions - conditions bright enough to see colors clearly - the rods are not functional, because they are all stimulated well past their response ceiling (and vision depends on the differences between the degrees to which different individual receptors are being stimulated). This means that, in daylight, peripheral vision is entirely dependent on the peripheral S-cones, distributed even more sparsely than are the rods. Daytime peripheral vision is thus even less acute than nighttime peripheral vision, and there is no color vision in the periphery, not even of blue things. Seeing colors depends on the differences in the amounts by which cones of the different classes are stimulated, and there is only one cone class out there. I think it often seems to people as though they can at least dimly see colors in the periphery, but this seems to be an illusion, dependent on memory. If you know that something you can sense in the periphery (because you have previously looked directly at it) is, say, red, you may get the impression that it still looks faintly red in the periphery. However, if some previously unseen object is introduced into someone’s peripheral vision, and they are prevented from looking directly at it, they cannot say what it is, or what color it is. They can only sense that something or other is moving over there.
Daytime peripheral vision provides very little discrimination of detail (worse, probably, than nighttime peripheral vision), and is really not good for much other than detecting unexpected movements, so that we can turn our eyes to look at whatever moved with foveal cone vision.
Does peripheral vision have a higher “refresh rate”? Sometimes I notice flickering bulbs at the edges of my vision, but not when I look at them.
I don’t think refresh rate is quite the way to put it, but, as I said, peripheral vision is very much adapted to detecting motion, which in practice means it will detect any fairly rapid changes in brightness at a position, so it makes sense that it might detect flickering better than the fovea can. I do not know if the phenomenon you mention has actually been studied or scientifically confirmed. (Very likely it has, but I don’t know.)