Rods, Cones, and Light - Video Tutorials & Practice Problems
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1
concept
Sensation of Light by Rods and Cones
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9m
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Video transcript
We now want to start thinking about how the retina actually converts light into vision, and to do that, we need to talk about 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 387 100 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 Rud 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 gonna be graphing on 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 gonna graph out the rods here. Alright. So rhodopsin absorbs best at a wavelength of 500 nanometers. If the eye is hit with a light of 500 nanometers, those rods are gonna give the maximal signal that they possibly can. Now to understand why rods can't see in color, remember we're only using 1 option. 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 give 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 gonna, cause the same amount of excitement in the rods. It's going to send the same amount of signal to 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 name: 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 gonna 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 gonna 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 gonna give about half the signal that it would at 420 nanometers for a light that's just as bright. But it's also gonna 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 next the medium wavelength or the m cone, sometimes called the green cone. So you can see here this medium wavelength cone absorbs a very different 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 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 gonna get, oh, roughly 50% of the total signal from the s or the short wavelength cone. But you're also gonna get signal from the medium wavelength cone, and you're gonna get even less signal from that long wavelength cone. That particular ratio of signal only exists on 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 gonna respond at all. It's not gonna give any signal. But if you go up, well, the medium and the long wavelength cone, they're gonna 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 longwave 9th 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 gonna 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 a black. All right. 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 gonna 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 gonna 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.
2
example
Rods, Cones, and Light Example 1
Video duration:
4m
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Video transcript
Our example here wants us to dissect this graph a little bit more, and it says, using the figure below, estimate what wavelength of light would cause such a reaction, and say what color the person would be expected to perceive. So we're going to look at 3 different responses, and for each one it says roughly how excited each of the different cones would be, the s, m, and l cones. Then we need to estimate a possible wavelength and a perceived color from looking at this graph. So reminder on the graph, on the x axis here, we have a wavelength from 350 nanometers up to 700 nanometers. And on the y axis, we have the relative response or relative absorbance of those different photoreceptors, so how much response we get from each wavelength. And we have a curve for the s cone, a curve for the rods, a curve for the m cone, and a curve for the l cone. Now the rods, we're talking about color here, so we can ignore this line altogether. Alright. First up, we have the response 1. It says the s cone is gonna be highly excited, the m cone is gonna have no reaction, and the l cone is gonna have no reaction. So look at these curves. Where on the curves does that look like it happens? Because as I look to get no reaction from either the M or the L cone, that really only happens way over here on the left side in these shortest wavelengths. So to get high excitement from this s cone and nothing from the others, I gotta go right about here. Right? That's pretty high excitement, and that I'm gonna estimate would be wavelength. So what does it look like? About 390, we'll say. And what color would you see there? Well, you would perceive that kind of as a blue or a violet, I'm gonna say. Alright. Next up, we have s cone is gonna have low excitement, m cone highly excited, and l cone mid excitement. So where do you see that? Low excitement for the s cone, high excitement for the m cone, and mid excitement for the l cone? Well, low excitement for s, so that's gonna be in this region here. And we're gonna be in the region where the m is really highly excited and the m cone is sort of mid excitement, that looks about there. So I'm gonna go ahead and draw a line on this graph that it's gonna be roughly there. That looks good. Maybe not quite a straight line, but you get the idea. Somewhere around there. So what wavelength could cause that? I'm gonna say that looks like oh, I don't know. How about wavelength 505 nanometers? And if your eye is hit with a wavelength of 505 nanometers, according to this graph, what are you gonna see? Person's gonna perceive something as green. Alright. Finally, we have s cone gets mid excitement, m cone gets mid excitement, l cone gets mid excitement. Take a look at the graph. Where does that happen? Nowhere. Right? You can't have that on this graph, but that can happen. So do you remember what we said happens when all these cones are getting sort of a similar signal? We said when all the cones are getting a similar signal and it's really bright, that's a white light. As it gets darker, it moves from gray to black. So because these are all mid excitement, I'm gonna say that you're gonna see a gray. And what wavelength are you seeing? Well, you're seeing many different wavelengths. It's not always that a single wavelength of light comes into your eye. Sometimes many wavelengths come in, and that's how we said you see colors that aren't perfectly on the spectrum. Okay. So again, remember I said you may need to break down a graph like this? It's probably more likely that you don't. But I really think that breaking down graphs like this really helps you understand how your color vision works in terms of that relative signal from the different cones. Alright. Like always, we got more videos coming up. I'll be there. Hope you are too.
3
example
Rods, Cones, and Light Example 2
Video duration:
5m
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Video transcript
In this example, we're going to talk about color blindness, and specifically we're going to say that redgreen color blindness is the most common form of color blindness, affecting roughly 8% of men. Now, just step back for a second. 8% of men is a heck a lot of folks. Right? So there are a lot of people out there not able to distinguish between greens, yellows, oranges, and reds. Maybe you're one of them. Alright? Let's keep going. We have mutations to the gene o p n one m w, and you can just read that sort of as opsin 1 medium wavelength, are the most common cause of red green color blindness. This gene codes for the MWS or medium wavelength sensitive opsin. If a person did not produce the MWS opsin, at what wavelength of light would they no longer be able to distinguish different colors? Use evidence from the image to support your answer. Alright. To figure that out, we gotta reorient ourselves to the image. So on the x axis, we're from 350 to 700. We have wavelengths in nanometers. We also have this color scale that aligns roughly what most people perceive color as to the wavelength that causes them to perceive it. And then on the y axis, we have from 0 to 100, relative absorbance. We have 4 curves on this graph, the s cone, the rods, the m cone, and the l cone. Alright. So straight off the bat, we're talking about color vision, so I don't care about the rods. Remember, rods have nothing to do with color vision, so I'm actually just gonna try and sort of white out this rod's curve. I don't care about it or what we're talking about. Alright. So get rid of the rods, and then I just want to start thinking alright. So for this typical type of red green color blindness, it's caused because the person doesn't make them medium wavelength sensitive cone or, I'm sorry, sensitive opsin. That means that their medium wavelength cone isn't going to be functioning. So we can basically also just erase this m cone. Right? So someone who has this sort of most typical form of red green color blindness, they're basically operating just with 2 cones in their eyes. They're operating with the s cone and the l cone. The m cone isn't doing anything for them because they don't produce that opsin. Okay. Now, to understand how this works, we got to think about how color vision works. So remember, we said to be able to see in color, you need signal from multiple cones at the same time. You need to measure that relative signal to be able to distinguish wavelength. If you only get signal from a single cone, you can't distinguish between intensity and wavelength. So now we gotta look where on this graph is someone going to get signal from more than 1 cone. Well, they're going to get signal from more than 1 cone, roughly from about here up to about here. So in that range of roughly, what's that? 400 up to about 540. Okay. So it's this region here that they get more than one signal, and people with typical red green color blindness will be able to see color in this range, those blues, cyans, up to greens, pretty similar, pretty much like anyone else can. But once you go over here on this graph, once you head up above 540, well, in this range, they're only getting signal from a single cone, only from that l cone. And if you get a signal from only one cone, you can't tell the difference between wavelength and intensity. People with red and green color blindness, right, they might see yellows, wavelengths, oh, I don't know, 570 here, 560. Those might look kind of intense or brighter. Reds might look dimmer, but they can't tell the difference in terms of color between those two things. So to answer our question here, at what wavelength would they stop being able to perceive color? Well, we said that's about wavelength 540. And my evidence is that from wavelength 540 to 700 nanometers and I should put my nanometers up here too. Nanometers. We're gonna say they get signal. There's signal from only the l cone. They're only getting signal from a single cone, and if you get signal from a single cone, you can't distinguish wavelengths, you can't see color. So people who are red green color blind, again, they have fairly typical sight or fair fairly typical color distinguishing from the blues into the greens, but then starting in the greens here into the yellows and the orange, they can see intensity, but they can't distinguish color. Alright. Hopefully, using this graph helped to make that a little bit more clear for you. We're gonna keep going with more practice problems after this. I hope to see you there.
4
Problem
Problem
A light hits your retina with a wavelength of 610 nm. Which cones will be stimulated, and what color will you perceive? Use the figure to help you answer.
A
S cone only; orange.
B
L cone and S cone; green.
C
L cone and M cone; orange.
D
S cone and M cone; green.
5
Problem
Problem
Imagine that you are walking in the woods at night in very low light. A red flower and a green leaf are both reflecting the same total amount of light. Which would appear brighter to you and why? The image is provided for reference.
A
The red flower. The cones will be most important in low light, and red wavelengths excite multiple cone cells.
B
The red flower. The rods will be most important in low light, and rods are most excited in the red spectrum.
C
The green leaf. The rods will be most important in low light, and rods are most excited in the green spectrum.
D
The green leaf. The cones will be most important in low light, and green wavelengths excite multiple cone cells.
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