Photoreception is the detection of light, and this is carried out by many different organisms. From us humans with our very complex eyes, to nematodes, you know, little roundworms in the soil that have photoreceptors that can detect the presence or absence of light. Now we're going to take a look at some more sophisticated systems that carry out photoreception, namely we're going to look at eyes. Now, insects and arthropods in general have an interesting eye called the compound eye. This is an eye composed of many little repeating units called ommatidia. And hopefully, you can see here all these little repeating units all over the eye. I mean, I can't even begin to dot them all. Those are all ommatidia, which are basically structures that contain clusters of photoreceptors.
Here we have what you could think of as a cross-section of the compound eye. So if we cut a slice into our compound eye, this stuff at the top represents the surface of the eye and these are all those subsurface structures. And, each one of these is an ommatidium.
And if we go over here, you can see a zoomed-in version of the ommatidia, and it's going to contain a cluster of photoreceptors, and that is how these eyes are going to detect light.
Now, the simple eye, which is just a single lens eye, is what we have. And this is going to operate similar to a camera, with its single lens. Now, the human eye can only perceive light in what's called the visible spectrum, which is just a small portion of the electromagnetic spectrum of electromagnetic radiation, which you can see down here. In fact, this visible light is not even close to proportional in this chart. The visible light is just a teeny little sliver on this huge range of different types of electromagnetic radiation.
Now looking at the eye, the white of the eye is known as the sclera. And this is basically a protective structure and it's going to be composed in part of collagen and elastic fibers to give it a tough resilient structure that can take a little squishing and motion.
Now, the cornea is basically the fluid-filled transparent cover over the iris and the pupil. It's kind of hard to see in a head-on image, but here you can see it as this area above the lens right here. Now, the iris is the colored area around the pupil, the pupil being this dark center of the eye, the iris is this region that I'm just kind of pointing out here. The iris is not just there to make our eyes look pretty and give them some color, it's actually there to control pupil diameter and lens shape, which is going to be very important for focusing light so that we can perceive a clear image.
Clearly, my eyes are incapable of that as I wear corrective lenses. I actually need a little assistance because my eyes can't quite focus the light properly.
Now, the pupil is that black hole as I said, and it's going to be the thing that allows the light to pass through the lens. So if you look over here, this structure is our lens and this opening here, that is on top of it, that is our pupil. It appears black because light is getting sucked in there, and black is of course what we see when there's an absence of light.
Now, the lens is going to actually change its shape to focus light from the cornea. However, the cornea itself has some strong refractive capabilities; we'll get into that in just a little bit. And essentially light is going to move through the cornea, through the lens, and then get focused onto the retina, which is the back of the eye that contains photoreceptors.
Let me jump out of the way here. So this area at the back of the eye is the retina, and light is going to enter cornea, get refracted through the lens, and project some image onto the retina. And that is where our photoreceptors are, and they will take it from there. So with that let's flip the page and actually see how those photoreceptors work.
