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Today is sensation. And specifically what we're going to look at is our sense of sight, so vision. So we know we have five senses. And each one of our senses requires two things. The first is some sort of physical stimulus. And in the case of vision that physical stimulus is light. And then the second thing that it requires is some sort of receptor. So some kind of specialized cell that can take the physical stimulus. So in the case of vision, that can take the light and convert it into a neural impulse. So the receptor in the case of vision is something called a photoreceptor. So we're going to go into the photoreceptor in a little bit. But first, let's just focus on the light. So what is light? Light is an electromagnetic wave that is part of a large spectrum. So there's something called the electromagnetic spectrum. And it contains everything from gamma rays and x-rays all the way to AM and FM radio waves. And so light just falls in the middle. And it ranges from something like violet, which has a wavelength of 400 nanometers, all the way to red, which has a wavelength of 700 nanometers. So the rest of the light that we see is somewhere in the middle over here. So basically a light is this electromagnetic wave that gets emitted from a bunch of different sources. So one of the most common sources, the most well known, is our friend, the sun. So the sun emits a bunch of light wave rays. These light rays come to earth and some of them go into our eyeballs. So let's go ahead and look at what happens when a light ray from the sun-- let's keep them right here-- comes down and hits an eyeball. So a little light ray comes in. And there's just a little guy standing over here. And let's just zoom in on his eye. So let's pretend this is his eyelid. Let's pretend he's looking right at the sun. You normally don't want to do that but let's go ahead and-- so this is the little pupil, the little hole in the eye in which the light enters. So this is the front of his head. This is the back of his head. So light comes into the eye and hits the back of the eye. So in the back of the eye there's this really special and interesting structure called the retina. So it just lines the back of the eye. It's this membrane. Just coats the very back of the eye. And it's composed of a bunch of different cells. And so inside the retina there are two really important cells. So let's go ahead and write the names of those two cells down. So one of those cells is called a rod. And it looks kind of like a rod. So it's got this rod shape. If you actually look at a microscope, looks like a little rod. And the other cell is called a cone. So inside the retina there are these two cells. And they're rods and cones. And it's called a cone because it looks like a little cone. And they're both smiling because they're happy. So basically these guys are all over this retina. So they're in there along with a couple of other cells that we'll touch on in a little bit. But basically they're really important. Because what they do is they actually take the light and they convert it into a neural impulse. So these are the big players. These are the receptors that we were referring to before. So let's talk a little bit about what the difference is between a rod and a cone. So rods have-- there are about 120 million of them. And they're really sensitive to light. So they're really, extremely sensitive to light. And they're really good for night vision. So when there's not a lot light out, they're really sensitive to it, so the little bit of light allows you to see at night. And they're also found all around the periphery over here. So there are a bunch of rods over here and over here in the periphery of your eye that allows you to see on the sides and allows you to see at night. There are a lot less cones. So there are about six to seven million cones per retina. And these cones, even though there are fewer of them, they're really important because they're responsible for color vision. So there are three different types of cones. There are red cones. There are green cones. And there are blue cones. So write blue cones. And we split them up into these three categories because red cones are really sensitive to red light. And green cones are sensitive to green light. And blue cones are sensitive to blue light. So there are these three main types of cones that absorb light that ends up being red, green, and blue. So the cones are centered in this little region of the retina right here. And we call that the fovea. So there are almost no rods in this part of the eye. And there are a whole bunch of cones. So much almost all the cones are centered in the fovea. And the fovea is basically the part of the eye that let's us see really fine details in pictures. So if you're searching for Waldo, the fovea is what let's you find him. So what happens now that the light enters the eye? It enters the retina. And it hits the back of the eye. What happens now? So basically the next thing that happens is something called the phototransduction cascade. So this phototransduction cascade is basically a set of things that occurs as soon as light hits a rod or a cone. So as soon as light hits this guy, the light wave triggers this phototransduction cascade. And we're going to go into the phototransduction cascade in the next video. But let's go ahead and skip over it for now. Just so that we can explain what occurs at the end of the phototransduction cascade. So let's go ahead and focus on just the rod for now. So here's a nice, happy rod. And light comes in from the sun, goes through the pupil, hits the retina, and then hits this little rod. So normally this rod is actually turned on. So when there's no light this rod is turned on. But when light comes and hits it, it actually turns the rod off. So when the rod is turned off, in a weird way it actually turns on this other cell over here, which is called a bipolar cell. So basically by the rod turning off when it's exposed to light, it actually turns on a bipolar cell. And the bipolar cell in turn, turns on another cell called a retinal ganglion cell. And this retinal ganglion cell basically goes into the optic nerve and then enters the brain. So, to the brain. So basically the process turning the rod from on to off is the phototransduction-- can't spell-- transduction cascade. So this is generally what happens when light hits the retina. It hits the rod, turns on all these cells, and then enters the brain. And then your brain goes and makes sense of what's happening by creating a rich visual field, which we can enjoy every single day.