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.
