What is the eye?

What does the eye do?

Now that we know some things that light can do, we’re ready to talk about your eye and how it encodes patterns of light in the environment.

First, let’s remind ourselves of what we saw light do:

1)   Light can be reflected off of a surface. What’s more, if we know the angle that the light made as it came towards the surface, we can also figure out the angle it will make when it leaves. In fact, those angles are equal.

2)   Light can be refracted, or bent, when it moves from one kind of substance to another at an angle. When I say “substance” here, I mean something like air, water, or the acrylic that the lenses we’ve been playing with are made of. Like reflection, we can also calculate the angles light makes as it refracts as long as we know some properties of the materials we’re dealing with (Figure 2).

3)   Finally, we also saw how light can be diffracted through small holes or slits. That is, light bends around corners in a way that depends on wavelength: Light with longer wavelengths spreads out more than light with a shorter wavelength. This allowed us to link long wavelengths to red light and short wavelengths to blue light.

All of these observations have helped us understand more about the physical nature of light. Now, what we want to do is begin to consider what light does when it interacts with the first stop in human visual processing: The eye. As always, let’s start with some observations: What is an eye? What parts does it have, and what do they do?

First, we can see one interesting feature of the eye just by looking at it. Your eye has a small dark opening that we call the pupil. Moreover, this opening changes when it encounters light: Shining light on it will make it smaller, while removing that light will make it larger (Figure 1). Whatever the eye is doing for us to help us see, this first step must have an impact.

Figure 1 - Shining light on your pupil will make it smaller. Remove the light and the pupil will grow larger again.

We can see a little more about what’s in the eye by using light to help us see some interesting things. Try the following: Close your left eye, and look to the left with your right eye. Now shine a small flashlight at the right corner of your right eye. You should briefly see a sort of network of branches flash across your entire visual field, and it may have looked something like Figure 2. What is all this stuff? These are small blood vessels that lie between the pupil and the part of your eye that actually senses light. Well, really, what you were seeing were the shadows of those blood vessels cast on the back of the eye.  For the moment, let’s just remember that these are here – we’ll use them later to explain a few things, but this will do for the present.

Figure 2 - Student drawings of the retinal vasculature they can see by shining a light through their sclera (the white part of your eye).

We can also see some other structures of the eye by casting shadows, but these take a little more work. Try looking at a bright patch of blue sky when you get a chance, and you may see something like Figure 3. These worm-like objects (usually called “floaters”) are small coagulated bits of a substance called the vitreous humor, which more or less fills the eyeball itself. We won’t talk much about these, but it’s another part of the eye that we can see in some circumstances.

Figure 3 - Looking at a bright blue sky often reveals "floaters," or bits of the vitreous humor that can cast shadows onto the back of your eye.

Finally, let’s get serious and take a proper anatomical look at the eye. Besides these structures that we can observe easily, there are also a number of parts that we can’t see so directly. These include the cornea, the crystalline lens, the Zonules of Zinn, and the retina. What are the functional consequences of all of these parts of our eye? That is, what do these different parts of the eye do to help us see light, and why do they have the form, location, and other properties that they have?

 Figure 4 - Cross-section of the eye. Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

We’re going to address this big question by building a physical model of an eye called a pinhole camera, and making some observations about how light behaves when it interacts with this model. To do this, you'll need to try the exercises in Lab #2.