7.4 The Eye

Critical Questions:

  • What does an eye do, and how does it work?
  • How do we achieve depth perception?

The complexity of the eye is frequently pointed to by creationists as proof that evolution didn’t happen, because it cannot possibly have resulted in such improbably effective organs. The argument goes something like this: “Just look at this thing. I mean, come on! What the heck?”

A hawk's eye, not to be confused with either Hawkeye the Marvel Avenger or the character from M.A.S.H. (Photo credit: Steve Jurvetson)
A hawk’s eye, not to be confused with the character ‘Hawkeye’ from M.A.S.H.
(Photo credit: Steve Jurvetson)

In fact, there is a pretty decent record of the evolution of the eye, from simple light-sensitive cells to the bafflingly awesome balls of goo we’ve all got stuck into our skulls. But despite this evidence, I suppose anyone might be forgiven for being impressed.

The eye has two important parts: one at the front, and one at the back. The front part is a lens, no different from a glass one – except that it’s made of living cells. The back part (the retina) is a wall of light-sensitive cells lining the inside of the eyeball. So the basics of the mechanism are fairly simple: light comes in, gets directed around by the lens, and is then detected by the detectors.

Note: the vitreous humor is not bright pink. [Source]
Note: the vitreous humour is not bright pink. [Source]
However, there’s a lot more to it than that.

The eyeball itself is mostly water. It’s approximately spherical, lined with muscles, and filled with a liquid that has a lovely name: vitreous humour. The main purpose of the muscles is to help us focus light. Just like in a camera, the light beams coming in through the pupil are spread out and need to be focussed on the back wall. That way, all the light coming from a single point in the outside world will meet at just one spot in the eye.

In other words, projected onto the inside back wall of your eye is an image of the world around you. (This image is, by the way, flipped upside-down, because of how lenses bend light.) Each of the photo-sensitive cells there sends information to the brain about what it detects – the average colour and intensity of the light hitting it – and the brain re-assembles all of this information into a visual picture.

To get the incoming light focussed correctly onto the back of the eye, muscles squeeze the eye into different shapes until the brain doesn’t detect any more blur, which is, of course, the product of unfocussed light. That’s why we tend to squint when we can’t see things well: squinting squishes the eyeball even more.

One complication here is that the brain receives signals from not one but two eyeballs. This means that they have to both work together in order to provide pictures that match up with each other. This is the secondary way we focus: our eyes swivel towards or away from each other until we see one coherent image.

As a secondary skill, our brains are also very good at judging distances of objects based on how our two eyes have to swivel to focus. This is the reason why seeing with only one eye makes the world appear less three-dimensional and more difficult to navigate.

Speaking of intelligent design, there’s one interesting fact about eyes that our own bodies try to hide from us. You might suppose that the veins that bring blood to the cells lining the backs of our eyeballs would be connected from the back, so that they wouldn’t get in the way of the incoming light. But you would be wrong. In fact, they’re connected from inside the eye, and they all meet up at one spot, where they exit the eyeball. This leaves us with blind spots at that location. But instead of leaving a big black spot in the middle of our field of vision, the brain actually fills it in with information from the cells around it. The only way to witness the blind spot for yourself is using bizarre and surprising activities like the one at this link. And if you want to go one step further and see the veins themselves, you can check out this YouTube video:

All right, that’s enough about the eye. You can go and read about rods and cones and stuff if you’d like, but you’ll have to do it somewhere else – we’ve got much more physics to discuss, and no time to waste on this biology nonsense.

Big Ideas:

  • The eye’s lens focusses light onto light-sensitive cells at the back of the eyeball.
  • These cells transmit the visual information to the brain, which uses it to build a picture of the distance and appearance of nearby objects.