Light is actively bounced out of the eye towards objects, so we can see
Keith S. Taber
Sophia was a participant in the Understanding Science Project. Y8 pupil Sophia had been studying sound and light in her school science lessons. Her model of sight involved light entering the eye, but then reflecting out again.
Do you know how you hear and see?
Does the light come in your eye, and it reflects off so you can see. … Just reflects, does it, and bounces on.
So if I've got my eye, some light comes in, some light comes in, what does it do, it bounces where?
About.
Inside the eye?
No around … it bounces out.
And then what?
Then you can see…so you look where you want to see, so it bounces off like in that direction, …you've got to actually look over, …you've got to look that way.
One long-established historical model of sight was based on rays coming from the eyes to detect objects in the outside world. Sophia's model appears to be a hybrid of this historical model, and modern understandings. For Sophia, light does not originate form the eye, but bounds out of it towards the object of sight. The idea that something must come out of the eye for us to see seems to be an intuitive assumption some people make – perhaps because we actually turn out heads and direct out eyes at what we want to focus on. This intuition has potential to act as a grounded learning impediment to learning the scientific model for vision.
But it's not quite that simple….
see https://www.npr.org/templates/story/story.php?storyId=96414364&t=1591812365995
"A lot of the animals we see, especially the ones that go out at night, have a special, reflective surface right behind their retinas," says Dr. Cynthia Powell, a veterinary ophthalmologist at Colorado State University. That light-reflecting surface, called the tapetum lucidum, helps animals see better in the dark.
When light enters the eye, it's supposed to hit a photoreceptor that transmits the information to the brain, Powell explains. But sometimes the light doesn't hit the photoreceptor, so the tapetum lucidum acts as a mirror to bounce it back for a second chance.
A large number of animals have the tapetum lucidum, including deer, dogs, cats, cattle, horses and ferrets. Humans don't, and neither do some other primates. Squirrels, kangaroos and pigs don't have the tapeta, either.
And not all eyes animals' glow the same color. Powell says this is due to different substances — like riboflavin or zinc — in an animal's tapetum. "Also," she says, "there are varying amounts of pigment within the retina, and that can affect the color." Age and other factors also can change the color, so even two dogs of the same species could have eyes that glow different colors.
So any light not absorbed on the first pass through the retina, is, so to speak, given a second chance of being detected. As a side effect, a small proportion of light entering the eye reflects out completely without being absorbed anywhere in the organ. Isn't science/nature full of wonders!
This gives the observed glowing eyes effect when a bright light shines on the eyes in otherwise low light conditions (so there is a high contrast). (Perhaps there is something of a parallel here to 'red eye' in flash photography – it seems anomalous and 'unnatural' in photos because under normal conditions the pupil appears black, but it is a natural effect of the high intensity of light illuminating the retina from the flash.)
So, in a sense, in these species some light IS emitted in the direction they are looking – although it is a by-product of the process of vision rather than part of the mechanism. But, yes, I can imagine a student learning about this effect (e.g., from a wildlife documentary) could misinterpret as light being sent out of the eye to enable the creature to see.
I seem to recall that the theory that vision was based on the eye sending out light was present in some pre-Renaissance Arabic texts, but I do not know if this phenomenon was known to the Arab scientists concerned: it may well have have been.