Question Video: Determining the Type of Lenses Used to Correct Farsightedness Science

Which of the following types of lenses can correct farsightedness? [A] Convex [B] Concave


Video Transcript

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.

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