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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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 cols. 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 option, we're gonna say is a protein pigment that absorbs light in the eye. And the different receptor cells in our eyes use different options. So to understand how this works, we'll start by talking about rods. Remember rods see in gray scale, they do not perceive color and rods use only one type of option. They use rod option. Now that's kind of convenient kind of easy to remember right. Rods use the option rod option. All right. So to understand how this works, so we're gonna be using this graph here. And so let me introduce it to you. You can see on the x axis we have wavelength and nanometers going from 350 nanometers all the way up to 700. And on the y axis, we have relative absorbents from zero all the way up to 100. What we're gonna be graphing on here is when a particular option 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 gonna 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. All right. So Rapson 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 one 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 uh cause the same amount of excitement in the rods is gonna send the same amount of signal. The brain. Your brain will say I'm getting about half the signal. Let's perceive a gray to understand how we see color. We need to use more than one option and that's how our cones work. So we're gonna say that our cones, well, we have three types and those three types are based on the option 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 options. Just remember the names of the cones. The options 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. All right. 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 gonna perceive color, we need to look at our other cones. So we'll do those one by one 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 uh 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 gonna 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? Right? 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 three at the same time. OK. 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 home. But you're also gonna get signal from the medium wavelength con and you're gonna get even less signal from that long wavelength con 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 gonna take that relative amount of signal, do some computations. And it's gonna say, oh, I see blue. All right. So we're gonna say here, color is perceived from comparing the relative amount of signal from all three cones. All right, 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 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 gonna 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 is something like uh 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 the almost the entire spectrum we see red because of the relative signal from all three cones, not just because that long wavelength cone is excited. OK. 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 three cones are excited equally. So if all three 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
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4m
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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 gonna look at three different responses. And for each one, it says roughly how excited each of the different cones would be the SM 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 wavelength from 350 nanometers up to 700 nanometers. And on the y axis, we have the relative response or relative absorbent 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. All right. First up, we have the response one, 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 was 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 and that's pretty high excitement. And that I'm gonna estimate would be wavelength though. 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, all right, next up, we have S cone is gonna have low excitement. M cone, highly excited and 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 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. Yeah, 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? No person is gonna perceive something as green. All right. 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 this spectrum. OK. 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. Like always got, 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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In this example, we're gonna talk about color blindness. And specifically, we're gonna say that red green, color blindness is the most common form of color blindness affecting roughly 8% of men. Now, just step back for a 2nd, 8% of men is a heck of 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. All right, let's keep going. We have mutations to the gene opn one MW and you can just read that sort of as option. One medium wavelength are the most common cause of red green color blindness. This gene codes for the MW SS or medium wavelength sensitive option. If a person did not produce the NWS option, at what wavelength of light would they no longer be able to distinguish different colors? Use evidence from the image to support your answer. All right, to figure that out, we gotta reorient ourselves to the image. So on the X axis with 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 absorbents. We have four curves on this graph, the S cone, the rods, the M cone and the L cone. All right. 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 rods curve. I don't care about it or what we're talking about. All right. So get rid of the rods and then I just wanna start thinking. All right. 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 option. 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 two 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 option. Ok. Now, to understand how this works, we gotta 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 one cone. Well, they're gonna get signal from more than one cone roughly from about here up to about here. So in that range of roughly uh what's that 400 up to about 540. OK. 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 blue cyan up to greens pretty similar or 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 green color blindness, right? They might see yellows wavelengths. So 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. Uh nanometers, we're gonna say they get signal, there is 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. Yeah. 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.
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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.
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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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