This lesson will examine the structure of the eye and will identify how our brain processes visual signals.

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Source: Human Eye; Public Domain: http://bit.ly/SewUbU VIDEO AND IMAGES CREATED BY AMANDA SODERLIND

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Welcome to this lesson today on vision. Today, we are going to be talking about the eye-- its structure and its function-- and the processes that happen that allow us to be able to see. So we're going to start today by looking at this diagram and labeling the various parts of this diagram and then talking about the function of each of these parts as we go.

We're just going to dive right in and start by labeling our lens, which is actually already labeled right here. OK, so the lens is the part of your eye that helps to focus incoming light. As light enters in through you eye, it'll actually enter in through your pupil, which is right in front of the lens, and then it will hit the lens. And then, that light will be reflected towards the back of the eye.

The pupil is, as I mentioned, the area of the eye where light enters. The pupil can constrict or dilate to control the amount of light that enters the eye. If it's really, really bright outside and you're outside on a really bright day, this pupil actually constrict to allow a little bit less light in. Or if you're in a really, really dim room, your pupils will dilate to try and let more light in, so that you can see a little bit more clearly.

Pupils allow light to enter through the eye and can control how much light enters in. And then, the pupil behind that is the lens, where that light is then focused back to the retina.

The next part that we're going to label here is the iris. And the iris is actually your colored part of your eye. If your eyes are blue, or green, or brown, or hazel, that colored part that you're seeing is called the iris.

And an interesting fact about the iris is that they're kind of like another set of fingerprints. The patterns and the colors and the variation in a person's iris are very unique to them. So it's a very individualistic part that can be used in some cases to identify a person, because they're very, very specific.

OK, the cornea out here is the next part that we're going to label. The cornea basically just covers the iris and helps to focus light. Then also in this front part of the eye here, we have aqueous humor. Aqueous humor is kind of this liquidity, gel-like substance that fills this space in front of the iris. It helps maintain pressure, and it also helps to transmit light to the lens as well.

So moving on from there, the conjunctiva is the next thing we're going to label. The conjunctive is basically just the very outer layer of the eye. Then within the eye-- within here-- we have our vitreous humor. The vitreous humor helps to maintain pressure, support the eye, helps the eye maintained its shape, so similar to the aqueous humor, but it's found behind the lens and obviously there's much more of it.

The sclera is the next part we're going to label. The sclera is the white of the eye. It's made of this dense, fibrous tissue that helps provide protection for the eye.

And then moving back now towards the back of the eye, we have the retina, which is all this part labeled here in yellow-- is the retina. The retina is where light is focused to. So light enters the eye and gets focused back to the retina.

The retina contains rods and cones, which are photoreceptors that help to process this incoming light and allow you to see images and colors. And we'll get a little bit more into those in just a moment. So basically, the retina's job is to absorb incoming light.

Then the fovea is this little area right here, if you can see this little indentation here. The fovea is an area in the retina where vision is the sharpest. And it's densely packed with all of these different types of photoreceptors. So that's the fovea . It's the area where vision is sharpest.

And then, we have the optic nerve. The optic nerve is the part of the eye that will then send information up to the brain for interpretation.

If we talk about how vision works, how do we actually see images? Basically what happens is, light will enter as a wave-- so light travels in the form of a wave. A light wave will enter through the pupil. The lens will then focus that light back to the retina. And then, rods and cones, or those photoreceptors found in the retina, will help to process that information.

Those rods and cones that are found within the retina-- those photoreceptors found in the retina-- are connected to neurons. Those neurons' axons form the optic nerve. The optic nerve can then send that information to the visual cortex of the brain for interpretation.

So those are kind of the different steps of how vision works. Light enters, is hit back to the retina, and then the retina helps to send brain to the optic nerve. And then from there, it gets sent up to the brain for interpretation.

In this part of the lesson, we're going to talk a little bit more specifically about rods and cones, which are those visual receptors that are found in the retina that we talked about on the last slide. Rods and cones contain visual pigments, which change their shape when different wavelengths of light are absorbed. Each color in the visible light spectrum-- if you're familiar with the visible light spectrum-- you know relates to a different wavelength of light.

So the wavelength of red or blue or green or purple or yellow are all different from one another. And these visual pigments change shape, depending on the type of wavelength that's absorbed. Visual pigments contain a version of the protein, opsin, plus retinal.

This retinal is actually derived from vitamin A. If you've ever heard someone tell you before that eating a diet high in carrots will help with your vision, there is actually some truth behind that. Because carrots are high in vitamin A, and retinal is derived from vitamin A. And retinal is a type of visual pigment. So you can kind of see how that's connected.

We're going to talk a little bit more specifically about the differences between rods and cones next. Let's first talk about rods. Down here, you can see the structure of rods-- what they look like. And this is kind of how they get their name, is because they kind of have this rod shape to them.

Rods are what allow us to see in dim light. And they allow for a more coarse perception. So if you were to walk into a pitch black room, right away you wouldn't be able to see much around you; it would be really dark. But these rods, after they can take a little bit of time to adjust, they would allow you to be able to see in that darkness. You'd be able to make out coarse objects, and you'd be able to see a little bit in that room.

Cones, on the other hand, contain a different visual pigment. So you can see the visual pigment for each of these. Cones contain a different visual pigment. And they allow for daytime vision and for you to be able to see in bright light.

There's actually three different types of cones, red, green, and blue. The variation of the visual pigment relates to whether it's a red, green, or blue cone. Cones allow for daytime vision. They allow you to see colors, they allow you to see bright light, they allow for better acuity, basically.

The reason that we can't see colors in the dark-- if you think back to our rods-- we can see coarse images in the dark. We can see this grayish, black colors-- is about all we can really see. And the reason that we can't see colors in the dark is because, in order to see color, we need cones. And cones need light in order to be able to process that color.

So if you were to walk, again, into a dark room and hold up two different color of shirts, you wouldn't be able to tell the actual color of either of them. Because in order, again, to be able to interpret color, you need to be in the presence of light. So that's the difference between rods and cones.

Those are the two different types of visual receptors that are found in our retina. These allow for dim light, coarse perception, daytime vision, bright light. And then, this is the difference in their shape. And that's kind of, again, how they get their name.

This lesson has been an overview on vision.

Terms to Know
  • Aqueous Humor

    Fluid within the eyeball that maintains pressure and transmits light.

  • Cone Cells

    A photoreceptor that detects bright light.

  • Cornea

    A layer that covers the iris and helps to focus light into the eye.

  • Fovea

    An area of the retina where vision is the most acute.

  • Iris

    The colored portion of the eye.

  • Lens

    A structure in the eye that focuses light to the retina.

  • Optic Nerve

    A nerve that sends impulses to the visual cortex in the brain where images are interpreted.

  • Photoreceptors

    Cells that detect light found on the basement layer of the retina; includes rods and cones.

  • Pupil

    The black part of the eye through which light enters.

  • Retina

    An area in the back of the eye that contains photoreceptors to absorb light.

  • Rod Cells

    A photoreceptor that detects dim light.

  • Sclera

    The white of the eye that aids in protection.

  • Vision

    The perception of light, images and movement by the eye.

  • Visual Cortex

    The part of the brain that receives nerve impulses from the optic nerve.

  • Vitreous Humor

    Fluid within the eyeball that supports the lens and the eyeball.