Human Physiology – Emmetropia, Myopia and Hyperopia

>>Dr. Ketchum: In this video, we will learn
about farsightedness and nearsightedness. Let’s begin with an emmetropic eye. Take
a look at this image. If you’re looking at something that’s far away, is that image
in focus for you? Is it sharp or a blurred image? Sharp, right, because your light rays
converge on a single vocal point, right? If you’re looking at something up close, is
that image blurred or sharp? Sharp. So if you have emmetropia, do you have good vision?
Yes you do. So this individual is going to see a sharp image, regardless to whether that
image is far away or close up. And that’s because when you look at these images, the
light rays converge onto a single focal point. So is this person going to need any kind of
corrective lenses? No, they have a perfect eye. All right, so now what we’re going
to do is we’re going to compare this normal eye to nearsightedness, which is called myopia.
So if you have a person that has myopia or they’re nearsighted, they have no difficulty
seeing objects that are near. But the problem is they have difficulty seeing objects that
are far away or distant. Here is a distant object. There is no corrective lens here.
Look at where the light rays are converging. Are the light rays converging in front of
the retina or behind? In front, right? So these light rays converge in front of the
retina. Where should the light rays be converging? The correct response is “On the retina.”
So the reason that this is happening is that the lens is too strong. So by saying too strong,
what does that mean about refraction? Do I get too much refraction or too little? Too
much, right? There’s way too much refraction, which causes the light rays to converge in
front of the retina. Not on the retina, but in front of it. So if you have myopia, your
focal length is what—too short or too long? Too short, so your focal length is too short.
What do you need to do to your focal length: increase it or decrease it? We need to extend
it, right? So in order to do that, we’re going to use a concave lens. Here’s your
concave corrective lens. By using that lens, what are the light rays doing when they pass
through the concave lens? They diverge right? So when you look at that image, see how the
light rays go out? The light rays are diverging because of the corrective lens. Then as they
pass through the cornea and pass through the lens, the light rays converge. So diverge,
then they converge as they pass through the cornea and the lens. So what has this done
to the focal length? It increased our focal length. So now our light rays are converging
where? On the retina. The whole problem with myopia is that the lens is too strong, and
therefore you have too much refraction—so you better account for that by having a concave
lens. So the light rays diverge, then the light rays converge as they pass through the
cornea and enter and exit the lens to increase your focal length. So now, in hyperopia or
farsightedness, you have no difficulty seeing objects that are far away but the problem
is that you have difficulty seeing objects that are nearby. Look at the two diagrams.
With a distant object, your light rays are converging onto the retina, which is perfect
vision! If you’re trying to look at an object that is nearby, where is your focal point:
in front of the retina, on the retina, or behind the retina? Behind the retina, correct?
So the focal point is behind the retina. And what’s the reason for that? Is your lens
too weak or too strong? Too weak. So is there enough refraction? No. So too weak of a lens,
not enough refraction. Is your focal length too long or too short? Too long. So what do
we need to do to the focal length? We need to shorten the focal length. So we’re going
to shorten the focal length by using a convex lens. Let’s focus on the convex lens. Does
that lens cause your light rays to diverge first, then converge? No. It’s going to
cause your light rays to converge. Okay, so this convex lens is going to converge the
light rays. So as the light passes through the cornea the light rays refract, and will
refract again when entering and exiting the lens. Now add in the convex lens and more
convergence occurs. Now your light rays are going to converge onto the retina. This decreases
the focal length. Again, if your focal length is too long, we shorten the focal length by
adding the convex lens so we have more refraction. All right, so the thing is, how are you going
to remember this? I’m going to give you a couple of little tools to help you. What’s
hyper mean? “Too much” or “in excess,” right? Look at where the light rays are at—they’re
too far beyond, or in other words, behind the retina. Farsighted means focal point is
too far. If you remember farsighted, focal point too far—if you think about that, this
will help you remember it. So you need to shorten it up. If you go back to myopia, now
you’re nearsighted, so your focal point is too short—it’s near. Nearsighted means
a near focal point, in front of the retina. So try to make those words give you hints
as to where that focal point is at. If you know where the focal point is then you’ll
know which kind of lens to use based on what you need to do to the focal length—if you
need to increase the focal length or shorten the focal length.


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