Lesson Video: Correcting Vision Using Lenses Science

Video Transcript

In this video, we will learn how human vision can be corrected using different kinds of lenses. We’ll learn about two different vision problems and see how two different kinds of lenses solve those problems.

As we begin, imagine we’re looking at a healthy human eye from this side-on perspective. When working properly, an eye can focus on objects very close, like this coin, or very far away, like this distant mountain. When rays of light from something close by reach our eye, the eye is able to focus these rays together. The inside surface of the back of the eye, colored here in pink, is called the retina. Whenever rays of light come to a focus on the retina, the image of the object being looked at, in this case the coin, is clear. The eye can see it sharply and distinctly.

For a healthy eye, not only can it bend light so that it can see objects nearby, it can also adjust to light coming from large, faraway objects much bigger than the eye, like our mountain, to bend these nearly parallel light rays too so once again they meet at the retina. A healthy eye is like an adjustable lens. It changes so that it can clearly see objects far away or objects nearby.

Unfortunately, not all eyes are healthy and able to do this. Some eyes have had their shape changed, a process called deformation. This eye is flatter and wider than a healthy eye. That changes the way the eye focuses light. If light from an object nearby enters this eye, the eye is still able to bring those rays to a focus on the retina. This means the nearby object is clearly seen.

But if light comes from an object very far away — here we can’t even see the object, but we assume it’s off the screen to the left — then this deformed eye focuses these rays so that they don’t meet on the retina anymore. We could say the eye overfocuses the light because the rays meet before they reach the back of the eye. Since the rays don’t cross at the retina, whatever the eye is seeing will look blurry. An eye with this kind of shape is called nearsighted. It’s able to see objects that are nearby clearly, but objects that are far away appear blurry and indistinct.

There’s another way that an eye can be misshapen or deformed. This eye is narrower and taller than a healthy eye. Once more, this different shape affects the way the eye focuses light. Light rays coming from a very faraway object are focused by this eye on the retina so it sees these objects quite clearly. But look what it does for light coming from an object very near the eye. These rays aren’t bent enough, we could say, to come to a focus on the retina. Because they don’t, nearby objects look blurry to this eye. An eye with this shape is called farsighted. It clearly sees objects far away, but not those that are near.

So a nearsighted eye sees clearly objects that are near but not those that are far away, while a farsighted eye clearly sees faraway objects but nearby objects look blurry. Nearsightedness and farsightedness are two kinds of vision problems. These problems can be fixed using lenses.

In general, there are two types of lenses: concave lenses with walls that curve inward, like this, and convex lenses that curve outward. If we look at what happens to parallel rays of incoming light when they reach a concave lens, we find that this lens spreads out the light. That is, the light rays travel farther away from one another after they pass through this lens.

Looking back at our nearsighted eye, this eye, we see, takes light from a faraway object and it bends this light, we could say, too much so that it focuses before the rays reach the retina. To correct this problem, we can put a concave lens in front of the eye. As we’ve seen, this lens spreads out the incoming light. Now, the eye can focus these rays from a faraway object on the retina. Even with this lens in front, a nearsighted eye is still able to see objects nearby clearly. The lens simply corrects the problem of faraway objects looking blurry.

Moving on to our convex lens, the other main type of lens, when parallel rays of light reach a convex lens, rather than being spread out, they are brought together. Knowing this, recall that the problem with the farsighted eye is that it wasn’t able to focus incoming rays of light from nearby objects enough. That is, the rays weren’t bent enough to focus on the retina. If we placed a convex lens in front of a farsighted eye, the lens would help focus light from nearby objects so that the eye could make these light rays cross at the retina. With the help of this convex lens, a farsighted eye can clearly see objects nearby. At the same time, this doesn’t cause any vision problems for faraway objects. Those objects can still be clearly seen by a farsighted eye.

Knowing all this, let’s look now at a few examples.

Which of the following is the term for the point at which light is focused by an eye with normal vision? (A) The lens, (B) the retina, (C) the eyeball.

An eye with normal vision has the overall shape of a sphere. This whole structure is called the eyeball. That’s answer option (C). When light enters a healthy eye though, it’s focused at a particular spot. The lens of the eye helps to focus incoming rays of light at the back of the eye. This whole surface along the inside of the back of the eye is called the retina. When light rays are focused on the retina, the object being looked at is clearly seen. We choose answer option (B).

Let’s look now at another example.

Which of the following types of lenses can correct farsightedness? (A) Convex, (B) concave.

Let’s start by recalling what farsightedness means for an eye. A healthy eye, one that can clearly see objects that are near as well as far away, is roughly in the shape of a sphere. It’s possible though for an eye to be deformed, to have its shape change. When an eye like this tries to focus on a nearby object, it’s not able to bend the light from that object enough so that the rays meet on its retina. This gap between the rays at the back of the eye means that whatever the eye is looking at will appear blurry. However, when an eye like this looks at an object far away so that the incoming light rays look like this, the eye is able to bring these rays to a focus on the retina. Therefore, the eye clearly sees the faraway object. This is why the eye is called farsighted.

In this exercise, we want to pick which type of lens, either convex or concave, can correct farsightedness. This means we want to know which kind of lens will fix this problem, the problem that a farsighted eye cannot clearly see objects that are near to the eye. Recall that a concave lens looks like this. The sides of this lens curve inward toward one another. When parallel light rays reach a concave lens, the lens causes the rays to spread out from one another. As we look at our farsighted eye though, notice that the rays are already too spread out, we could say. To bring them to a focus, we want them to meet at the retina. That is, we want to bend them more towards one another, not less. To do this, we can use a convex lens. This type of lens takes parallel rays of light and it bends them together so they come to a focus.

If we put a convex lens in front of a farsighted eye, the lens helps focus light from a nearby object. So the eye is able to bend those rays and have them meet at a point at the back of the eye on the retina. With the addition of the convex lens, this farsighted eye can clearly see objects nearby. And note that it can still clearly see faraway objects. We choose answer option (A). A convex lens can correct farsightedness.

Let’s look now at one last example.

Which of the following types of lenses can correct nearsightedness? (A) Concave, (B) convex.

Let’s first remember what it means for an eye to be nearsighted. When an eye is healthy, it takes on roughly the shape of a sphere. It’s possible though for an eye to be misshapen or deformed. This changes the way the eye focuses incoming rays of light. When a nearsighted eye, like this, looks at an object that’s very near the eye, the eye is able to bend those incoming rays of light so that they meet at the back of the eye, called the retina. When this happens, the up close object, whatever it is, is clearly seen.

The problem with a nearsighted eye shows up when it looks at objects far away. A nearsighted eye bends these incoming rays so that they cross before they get to the back of the eye. Because the rays are not focused at the back of the eye, the faraway object looks blurry.

The vision of a nearsighted eye can be corrected using a lens. The two main types of lens are convex and concave. A convex lens brings parallel incoming rays to a focus, while a concave lens spreads out incoming parallel rays. So let’s think. With our nearsighted eye, light from objects far away is focused so that the rays meet before they reach the back of the eye. We could say, in other words, that this eye focuses light from faraway objects too much.

To move this focus point to the back of the eye, we could put a concave lens in front of it. This way, light coming in from a faraway object is spread out so that when the nearsighted eye focuses these rays, they do meet at the back of the eye, at the retina. Putting this concave lens in place in front of the nearsighted eye means the eye can even see faraway objects clearly. For our answer, we choose option (A). A concave lens can correct nearsighted vision.

Let’s finish this lesson now by summarizing a few key points. In this video, we saw that a normal eye is able to clearly see objects far away and nearby. It does this by focusing light from those objects on the back of the eye, called the retina. A normally shaped eye can be deformed in two ways. One sort of deformation creates an eye that can clearly see objects nearby but can’t clearly see objects far away. This eye, therefore, is called nearsighted. On the other hand, an eye with a different deformation is called farsighted. This eye can clearly see objects far away, but objects nearby look blurry.

Both farsighted and nearsighted vision can be corrected using lenses. A concave lens corrects nearsighted vision. This means a nearsighted eye can clearly see objects far away. On the other hand, a convex lens corrects farsighted vision. It does so by allowing a farsighted eye to clearly see objects nearby. This is a summary of correcting vision using lenses.