>>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.