In this explainer, we will learn how to use different kinds of lenses to correct human vision.
When our eyes are working properly, we see objects around us clearly.
The objects might be nearby, such as a coin within arm’s length, as shown below.
The objects might also be far away. Consider the diagram below, which shows how light rays from an increasingly distant mountain reach our eyes.
This diagram shows that the farther from the mountain the eye is, the smaller the angle between the rays from the base and the top is, and the more nearly parallel these rays are.
At a great distance, this is approximated by the following diagram.
Objects look clear (not blurry) to us when our eyes are able to focus the light coming from them. Notice that the eye does not need to bend light rays from faraway objects (like the mountain) as much as it needs to bend light coming from objects nearby (like the coin) in order to see them clearly.
In a healthy eye, a part of the eye called the lens focuses incoming light on the back of the eye, as shown below.
At the back of the eye is a part called the retina. When light is brought to a focus on the retina, as shown above, we see a clear, sharp image.
Example 1: Naming the Point Where Light in a Healthy Eye Focuses
Which of the following is the term for the point at which light is focused by an eye with normal vision?
- The eyeball
- The retina
- The lens
In an eye with normal vision, incoming light is focused by the lens of the eye. The lens is near the front of the eyeball. Once light passes through the lens, it is focused to a point on the back of the eye. This part is known as the retina.
The term for the point at which light is focused by an eye with normal vision is the retina, answer option B.
Example 2: Identifying the Position of the Retina in the Eye
Which of the following is the position of the retina in the eye?
- At the center of the eye
- At the front of the eye
- At the back of the eye
The retina is where light in the eye is brought to a focus. This happens after incoming light passes through the lens near the front of the eye.
The lens is adjusted so that it focuses light to a point at the back of the eye. The back of the eye is covered by the retina. Therefore, we choose option C as our answer.
So far we have considered only healthy vision, where the lens is able to focus incoming light on the retina at the back of the eye.
Proper vision depends on the shape of the eyeball. If the eyeball shape becomes irregular, the lens may no longer be able to focus light on the retina.
The following diagram shows a healthy eye (on the left) alongside a misshapen eye.
Notice that the light entering the misshapen eye comes to a focus before it reaches the retina. As the diagram shows, once this light reaches the retina, it will not be in focus. The result is a blurry, distorted image.
An eye that focuses light to a point before the light reaches the retina is said to be nearsighted. This name is given because a nearsighted eye can bring into focus objects that are nearby, but not objects that are far away.
Nearsighted eyes focus some incoming light too much, bending it so the rays meet before they reach the retina at the back of the eye. Other eyes, however, focus incoming light too little so that the rays of light do not come to a focus at all inside the eye. Consider the diagram below.
The eye on the right is narrower and taller than the normal eye. Because of this eye’s shape, it is unable to focus some light rays to a point inside the eye. Once again, the image formed is blurred.
The narrower eye is said to be farsighted. Farsighted eyes are able to produce clear images of faraway objects, while objects nearby look fuzzy and indistinct.
Nearsighted eyes and farsighted eyes both suffer from limits on how much the eye’s lens can focus light.
These limits can be changed by using lenses—such as those in a pair of eyeglasses—outside the eye.
In general, a pair of eyeglasses might have two types of lenses. One type makes incoming light rays spread apart, while the other type makes those rays focus to a point.
A lens that spreads out incoming light is called a diverging or concave lens. An example of a concave lens is shown below.
By diverging incoming light, a concave lens can correct a particular vision problem.
Recall that nearsighted eyes focus light too much, making light rays from distant objects focus to a point before they reach the retina.
By placing a concave lens in front of a nearsighted eye, incoming light rays are first spread out (by the concave lens) and then focused (by the lens in the eye). The result is a set of rays focused at the retina, as shown in the following diagram.
Putting a concave lens in front of a nearsighted eye can correct the eye’s vision problem. It is now able to focus light rays from faraway objects onto the retina, producing clear images.
While concave lenses are one type of lens we might use for eyeglasses, the second common type are called convex lenses. Convex lenses focus light rays to a point, as shown below.
Convex lenses can correct the vision of a farsighted eye. Recall that a farsighted eye is unable to focus light from nearby objects on the retina.
However, if a convex lens is placed in front of a farsighted eye, this problem can be corrected as follows.
Example 3: Choosing the Lens That Corrects Farsighted Vision
Which of the following types of lenses can correct farsightedness?
A farsighted eye is unable to see objects nearby clearly. This is because a farsighted eye cannot focus light rays from nearby objects enough to meet at the eye’s retina.
The vision of a farsighted eye can be corrected by putting a lens in front of the eye that helps focus light.
The type of lens that focuses light, rather than spreading it apart, is called a convex lens. This lens can be paired with a farsighted eye to produce clear vision.
We choose answer option B.
Example 4: Choosing the Lens That Corrects Nearsighted Vision
Which of the following types of lenses can correct nearsightedness?
A nearsighted eye is unable to see distant objects clearly. This is because a nearsighted eye focuses light rays from distant objects too strongly, causing them to meet before reaching the eye’s retina.
The vision of a nearsighted eye can be corrected by putting a lens in front of the eye that helps spread out incoming light rays.
The type of lens that spreads out light, rather than focusing it, is called a concave lens. This lens can be paired with a nearsighted eye to produce clear vision.
We choose answer option A.
- In a properly working eye, the eye lens focuses light rays on the retina, located at the back of the eye. Images formed on the retina are clear and distinct.
- If the eye is misshapen, it may focus light rays so they meet in front of the retina or behind it.
- An eye that bends some incoming light rays so they do not meet inside the eye will produce clear images of faraway objects but blurred images of nearby objects. Such an eye is said to be farsighted.
- An eye that bends some incoming light rays so they meet in front of the retina light will produce clear images of nearby objects but blurred images of faraway objects. Such an eye is said to be nearsighted.
- Convex lenses focus incoming light and can therefore correct farsighted vision.
- Concave lenses spread out incoming light and can therefore correct nearsighted vision.