We now want to start thinking about how the retina actually converts light into vision, and to do that, we need to talk about the sensation of light by the rods and the cones. And so I like to just start by reminding ourselves that light is just electromagnetic radiation with wavelengths that are between 387 and 100 nanometers. And I like to remind myself that there's nothing special about light that makes it different from X-rays or radio waves other than the length of those wavelengths. And we're able to perceive light because we're able to sense those wavelengths, and we're able to perceive color because we're able to distinguish between different wavelengths. Now we're able to do all that because of a very special protein in our eye. That protein is opsin. Opsin, we're going to say, is a protein pigment that absorbs light in the eye, and the different receptor cells in our eyes use different opsins. So to understand how this works, we'll start by talking about rods. Remember, rods see in grayscale; they do not perceive color. And rods use only one type of opsin: They use rod opsin. Now that's kind of convenient, kind of easy to remember, right? Rods use the opsin rod opsin.
Alright. So to understand how this works though,we're going to be using this graph here. And so let me introduce it to you. You can see on the x-axis, we have wavelength in nanometers going from 350 nanometers all the way up to 700. And on the y-axis, we have relative absorbance from 0 all the way up to 100. What we're going to be graphing here is when a particular opsin is hit by a particular wavelength, how much of that wavelength does it absorb on a scale of 0 to 100? And how well it absorbs that wavelength is going to tell us how well the cell responds, how much signal is actually sent to the brain. So to see this, let's start. We're going to graph out the rods here. Alright. So, rhodopsin absorbs best at a wavelength of 500 nanometers. If the eye is hit with light of 500 nanometers, those rods are going to give the maximal signal that they possibly can.
Now to understand why rods can't see in color, remember we're only using 1 opsin. So as you get away from that 500 nanometers, it's giving less signal, but your brain isn't able to tell whether that's because it's not absorbing light as well or maybe because the light in it isn't as bright. Right? So let's look, for example, at 450 nanometers. At 450 nanometers, a wavelength of that light hitting your eye will excite the rods about half as much as they possibly could be excited. But your brain doesn't know, am I getting sort of half the signal because it's a wavelength of 450 nanometers, or am I getting half the signal because it's a wavelength of 500 nanometers that's just half as bright? Both things are going to cause the same amount of excitement in the rods. It's going to send the same amount of signal to the brain. Your brain will say, "Oh, I'm getting about half the signal. Let's perceive a gray." To understand how we see color, we need to use more than 1 opsin, and that's how our cones work. So we're going to say that our cones, well, we have 3 types and those 3 types are based on the opsin that's used. We have the short wavelength, medium wavelength, and the long wavelength. And here, you don't really need to worry about the names of the opsins. Just remember the names of the cones. The opsins have the same names: the short wavelength, medium wavelength, and the long wavelength.
All right. So let's start with this short wavelength cone. It's also sometimes called the S or the blue cone. Now, I don't like calling them by their color names, the blue, green, and red cones, because I think that confuses how color vision works. But you are very likely to see that, so you should be familiar with them. So let's graph this S or short wavelength cone here. Before we do that, I'm just going to put a color scale here, and this is how most people perceive these different wavelengths. So at about 400 nanometers, most people are seeing a blue or purple. All the way up to 650 or 700 nanometers, you're seeing a red. Alright. So if we graph this short wavelength cone, you can see it absorbs best at 420 nanometers there, and it absorbs not nearly as well as you get away from that sort of narrow peak. We're going to have the same problem that we had with rods though if we're just using a single cone. Let's look again at 450 nanometers here. At 450 nanometers, this cone is going to give about half the signal that it would at 420 nanometers for a light that's just as bright. But it's also going to give half the signal if it's hit with a light that's 420 nanometers, that's just half as bright. So to understand how we're going to perceive color, we need to look at our other cones. So we'll do those 1 by 1. First, we have the medium wavelength or the M cone, sometimes called the green cone. So you can see here this medium wavelength cone absorbs a veryldifferent spectrum. It has a peak roughly around 535 nanometers, but it's absorbing from 400 up to something like 650 here. Quick note, you probably don't need to know those exact numbers, just how to interpret a graph like this. So as we look here, this is really absorbing a whole bunch of the spectrum, but it's going to still have that same problem. How do we know if it's absorbing a wavelength that isn't as bright or a wavelength that it doesn't absorb as well? Alright. Let's look at the L cone or the long wavelength cone, sometimes called the red cone. And here we can see it's a really very similar shape to that medium wavelength cone. It's just sort of shifted over a bit. It's still absorbing most of the spectrum. Now it has a peak at roughly 565 nanometers, but it's absorbing from roughly 410 to 700 nanometers. So to understand how we see in color, we need to look at all 3 at the same time. Okay. Now let's look at that same wavelength, 450. So if you follow 450 up, you're going to get, oh, roughly 50% of the total signal from the S or the short wavelength cone. But you're also going to get signal from the medium wavelength cone, and you're going to get even less signal from that long wavelength cone. That particular ratio of signals only exists at one point in this graph, 450 nanometers. And if your brain gets hit with a wavelength of 450 nanometers, it's going to take that relative amount of signal, do some computations, and it's going to say, "Oh, I see blue." All right. So we're going to say here color is perceived from comparing the relative amount of signal from all 3 cones. Alright. To practice this one more time, let's look at, oh, I don't know, 545 nanometers. Now here, well, that short wavelength cone isn't going to respond at all. It's not going to give any signal. But if you go up, well, the medium and the long wavelength cones, they're going to give just about the exact same amount of signal. That ratio, no signal from the short wavelength cone, the exact same amount from the medium long wavelength cone, only happens at one point on this graph. 450 nanometers, what your brain will perceive as green or a yellow screen. So that's going to be true of all the different wavelengths. The relative signal is going to be unique. It only exists at that particular wavelength. Now to quickly see why I don't like calling them these cones by their color names, Well, let's look at the red cone or the long wavelength cone over here. Well, red is way out here, but it doesn't actually absorb red wavelengths very well at all. It absorbs best at 565 nanometers, which is what we see as something like yellow. Right? We call it the red cone because it's the only cone that absorbs red, but it's actually not very good at absorbing red. It absorbs almost the entire spectrum. We see red because of the relative.signal from all 3 cones, not just because that long wavelength cone is excited. Okay. Now you may say, "Well, what happens when we get more than one wavelength coming into our eye?" Light is often a mix of wavelengths. Absolutely. That's how we see colors that aren't perfectly on this color scale. That's how we could see brown or pink, for example, or white. White, we're going to say, is when all 3 cones are excited equally. So if all 3 cones are excited equally because there's lots of different wavelengths coming into our eye and it's really bright, we'll see a bright white. If they're excited equally but not that bright a light is coming in, we'll see a gray. If they're excited equally but no light is coming in, we'll see black. Alright. You're very likely to see a graph very similar to this. Whether or not you have to break it down in this way on a test is going to depend very much on the class that you're in. But I think regardless of whether you have to do it on a test, being able to break down a graph like this really, really helps to understand how color vision works. So we're going to practice some more. We'll even see an example where we understand how red-green color blindness works based on this graph. I'm looking forward to it. I'll see you there.
