The Human Eye

by Jared Rovny

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    00:01 In terms of the human eye, let’s explore this idea of magnification a little bit more.

    00:06 We call for the near point of a human eye 25 centimeters. This is the closest an object can still be in focus easily for the human eye. As you bring an object closer and closer, your eye will have more and more trouble actually focusing on the object since your eye is also a human lens. So, for the magnification for something for a human eye, what we would say is it is again the ratio of the angle, where the angle is how big an angle in your vision the object is taking up. But instead of the output angle that you'd get from your instrument divided by any input angle, we define the magnification relative to the angle for this near point, so θ at 25 centimeters away from your eye since this is the near point for the human eye. Let’s talk a little bit more about the eye itself and see how the workings of the eye cause you to see an image. All the light from a particular point in space when it comes in towards your eye will pass through the lenses of your eye and will try to be focused towards the retina. All these points at a particular point in space should hopefully all match up to a particular point on your retina. For example, if you’re looking at an object in your room, one point on that object again will be sending light in many different directions as we already discussed. Much of that light will reach your eye and be focused by the lens. The goal of the lenses to move the eye is to try to get all of those rays to point to a particular spot on your retina if you’re focusing on that object at a particular distance. Things are usually or often at least far away enough from your eye that we can consider the rays coming into your eye to be parallel rays. This is almost always a good approximation. The only time that your eye, for this reason, will have to use its muscles and stretch this lens is if those objects are too close. Then in that case, those rays are not coming in parallel towards your eye but are coming in at some sort of angle because we can assume that they’re so far away that the light rays are coming in parallel to each other.

    02:02 If this is the case and the light rays are coming in at some angle instead of parallel to each other, your eye will need to change its lens structure. It does this by using muscles on either side of the lens of your eye in changing to geometry of the actual lens of your eye. Sometimes this doesn’t always work. We have light rays coming in. Maybe these rays, as they come in, instead of focusing on the retina as they’re supposed to, maybe focused too soon or too late. So, we can always use corrective lenses or in other words, glasses or something like them to change the way that these light rays are coming in. For example, in the eye that you can see here, on the top image, it’s focusing those rays too close. They’re not hitting the retina properly. What we could do is first diverge those rays a little bit so that when they hit the lens of your eye, they’re going to converge slowly than they ought to but they’ll still hit the right spot because we’ve picked the right divergence of the lens on the bottom here to make sure you can see the objects that you’re looking at.

    03:00 This concludes our basic overview of some of the optical instruments. There’s not a lot of detail here but there’s certainly much detail to be learned about optical instruments if you’re interested.

    03:09 You’ll just need to be able to know the basic workings, the basic functions of lenses, whether they’re converging or diverging and how those lenses work in something like the human eye or the basic idea behind how these lenses work in a telescope or microscope rather than needing all those details. But for the lecture before this one, again pertaining to optics about lenses and mirrors and how to find images, you’ll definitely want to do a lot of practice to make sure you’re very comfortable with the parallel rays, the focal points, and how to find images and objects as well as their size and the magnification for the lenses or the mirrors that you're talking about. Now, we’re going to move on to some different topics. We’re going to zoom in to a much smaller world. Until then, thanks for listening.

    About the Lecture

    The lecture The Human Eye by Jared Rovny is from the course Geometrical Optics.

    Included Quiz Questions

    1. Converging lens
    2. Diverging lens
    3. Convex lens
    4. Concave lens
    5. Mirror
    1. When an object is close enough, and the rays are no longer approximately parallel
    2. When an object is so far, and its rays start to become approximately parallel
    3. When an object is so far, and its rays stop being approximately parallel
    4. When an object is so close, and its rays are approximately parallel
    5. The lens must always be bent
    1. Light is initially bent to compensate for the incorrect bending of light by the eye’s lens
    2. The eye’s lens is forced to compensate because of the glasses
    3. The glasses have a higher index of refraction than the air, causing the light to reflect and refract partially to compensate for the eye
    4. The glasses cause light to converge, allowing more light to reach the eye
    5. The glasses focus light to the lens of the eye itself, hence allowing the eye’s lens to focus the incoming light onto the retina consistently

    Author of lecture The Human Eye

     Jared Rovny

    Jared Rovny

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