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- Okay, so we're still talking about how to recover a pattern, how to recover an image of things or objects out in the world. We've talked about tubes, we talked about pinholes, we talked about lenses. And now another strategy is mirrors. These are very rare in nature, but they do exist. You can use mirrors to focus light, just the way you use lenses to focus light, only mirrors work in a slightly different way. Here's a fish called the Brown snout spook fish, which has light enter the eye, reflects that light off of a concave mirror surface, and that light is then focused onto a membrane that has photosensitive units are photosensitive cells. So the concave mirror here can focus light.
- Now, this is a principle that's very frequently used in astronomy, telescopes, in cameras. But it's not so commonly used in the natural world. Sometimes it is. And this is one pretty striking interesting example of that. Now once light is recovered via any of these strategies, tubes, pinholes, lenses, or mirrors, the light needs to be used by the organism, whether it's us or your dog.
- And to do that, we need to transduce the light transduction is the conversion of light, electromagnetic radiation into electrical and chemical signals that the brain can use to represent information. Photoreceptors serve that function.
- Photo receptors transduce electromagnetic radiation, electrical and chemical signals. Now here's some creatures, really early creatures that have accomplished this through evolution. Amoebas and Paramecium, or single cell creatures that don't have any lenses. There's no pinhole here. There's just one receptor, but they are photosensitive, so they've managed to convert electromagnetic radiation into other kinds of signals, electrical and chemical signals that they can use, for example, to guide movement. Amoebas can be photo negative, for example, but a negative kidneys just means move away from the light paramecium to photo positive kinesin. In other words, they can move toward the light. To light can change behavior in this case. And that's because light is converted or transduced into electrical and chemical signals. Here's a more complicated creature, a limpet or a flat worm. This is a particular kind that has, I dimples rate here. These are so-called eyes. Really, it's just a temple with a set of cells that are photosensitive. These photosensitive cells are located in this simple, they're not on a flat surface. Now why is that useful?
- Okay, so that's accommodation. Now, what the eye is doing is focusing light on the back surface of the interior of the eye, and that's called the retina. And this is a key structure. This is again, the cornea is the front surface. That's what does most of the focusing. Then you have a liquid like structure, liquid like substance in here. It's almost, it's watery like then you have the lens with the pupil opening. Light travels through that through the lens and it's focused on the back surface of the eye. Back surface of the eye. Inside the eyes, the retina, that's a surface that's covered with cells. And these cells are called photoreceptors. Here's the inside of an eye. This is the inside of my eye actually. And you can see reddish. There are lots of game-like structures going on to reddish NIS. It's from blood feeding the, feeding all those photoreceptors feeding other cells. You can see the veins, the vascular structure. All of those veins and vascular structure sits on the front of the retina. In other words, in front of the photoreceptors, the light has to get through them to get to the photoreceptors.
- So here's the retina. Here's a cross-section of the retina in cartoon form. The retina has this cross section where the very back of the retina or the photoreceptors. In front of those photoreceptors are a bunch of layers of cells, different kinds of cells. There are a series of cells that were, some of which we're going to talk about later. We're not going to talk about them at this moment.
- First, we're just going to talk about these photoreceptors. And we're going to talk about the fact that all of these other cells, the stuff that's junk, and some vasculature, some blood vessels sit in front of these photoreceptors, so the line has to pass through them to get to the photoreceptors. Seems like a backward construction. But it turns out that there are some good reasons for it.
- Now the photoreceptors receive the light and they transduce it, convert it from electromagnetic radiation into electrical and chemical signals that the brain can use. Those chemical and electrical signals are collected by the photoreceptors and collected by these cells in front of the photoreceptors and then sent out the eye optic nerve. The optic nerve is the hole in the back of the eye through which the fibers exit. That's like the, it's like the fiber optics at your house. They have to get outside your how somehow to bring the cable in the hole and the eye is called the optic nerve. And it sits right there in this region, right there at the optic nerve is called the optic disk. Ok?
- So these photoreceptors consists of two types, their rods and there are cones. For us, for humans, there are rods and cones. Now, there are some key differences between the structure and the function of rods and cones. This is a rod, for example, right here, it's rod-like.
- Cones are bigger and they tend to be cone shaped. Quite a bit bigger often than rods. You can see rods are these little dots here. This is the back of the eye, the retina photograph taken of the surface. If somebody's retina, the cones at the big things here, the rods at the small things here. Well, it's tend to be smaller and they tend to be rod-shaped. That's a picture of a rod on right? Now, there are three types of cones, but there's only one type of rod.
- The cones serve our color vision. There are so-called blue cones, green cones, and red cones. These cone photoreceptors are not actually blue, green, and red. They are just sensitive to different ranges of electromagnetic radiation. The red cones are really better, better known as long wavelength cones because they're sensitive to relatively long wavelengths of light. The green cones are really medium wavelength cone, so we might call them M cones are medium wavelength flight because they're sensitive to mid-range of light that happens to be centered around the greenish territory. The blue cones are really short wavelength sensitive counts. We might carlos S cones. And that's because they're sensitive to short wavelengths of light. The rods are sensitive to a range of wavelengths, sort of in-between the blue and green cons, the short and the medium wavelength cones. These rods are sensitive to, most sensitive to light in the 450 to 500 nanometer kind of range.
- So different photoreceptors are sensitive to different ranges of wavelengths.

- Convergence of electromagnetic energy into chemical energy. Early organisms (amoeba proteus and paramoecium) were photosensitive and responded to the light. More complex: flatworm, limpit. Pinhole camera in nature: chambered nautilus.