Video: Understanding the Construction of a Concave Lens

Which of the following statements correctly describes a concave lens? [A] A concave lens is thicker than a convex lens. [B] A concave lens has uniform thickness. [C] A concave lens increases in thickness from its centre to its edges. [D] A concave lens decreases in thickness from its centre to its edges.

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Video Transcript

Which of the following statements correctly describes a concave lens? A concave lens is thicker than a convex lens. A concave lens has uniform thickness. A concave lens increases in thickness from its centre to its edges. A concave lens decreases in thickness from its centre to its edges.

Looking over these four statements, we see that they’re all different claims about a concave lens. Concave lenses are one common type of lens. Another, named in one of our answer options, is a convex lens.

Now say that we have a lens that looks like this. We can see that if we look at the lens with our eye over here to the left of it, then the lens is bulging out towards us, toward our eye. On the other hand, if our vantage point is on the other side of the lens, then in this case, we see the lens bowing inward.

Thinking about looking at a lens like this from either side as we have here is one helpful way to remember the difference between these two common types of lenses, concave and convex. While a convex lens is one that bulges out in the middle like this side here. A concave lens is one whose top and bottom seem to curl around its middle portion. We can remember the distinction between these two by thinking of the word “cave” in concave.

A cave is something that we can walk into. And the way that this lens curves when our eye is on the right side of it over here takes on a shape that can remind us of this. All this means that if we were to draw a lens which is specifically concave, that lens would look something like this. Regardless of the side from which we view this lens, in both cases, it curves inward from either side. In that sense, it’s like a cave we can walk into, which reminds us that it’s concave. In contrast to this, a convex lens would look something like this. It bulges outward when viewed from either side.

Now that we know what a concave lens would look like in general, let’s reconsider our four answer options for describing this lens.

The first option says that a concave lens is thicker than a convex lens. Well, the concave and convex lenses we’ve drawn here seem to suggest this relationship. But it’s not necessarily always the case that a concave lens is thicker than a convex lens. For example, what if our convex lens looked like this, where it’s practically a sphere? In that case, the convex lens is the thicker of the two. So we won’t choose option A for our answer.

The next option says that a concave lens has uniform thickness. This description of uniform thickness means that if we drew a vertical line down through the middle of our concave lens. Then the distance from that line to the outer portion of the lens on either the left or the right side will always be the same. But we can see that that’s not the case. That the lens has a greater thickness towards its edges than it does towards its centre. So we’ll eliminate this option as well.

Our next answer candidate says that a concave lens increases in thickness from its centre to its edges. So let’s think about what this is saying. It’s saying that the thickness of the lens at its centre, right here, is less than the thickness of the lens at its edges, at its top and bottom. And based on our diagram, this seems like a true description of this lens. We’ll put a check mark by this option, keep it in mind, and consider our last option, choice D.

This says that a concave lens decreases in thickness from its centre to its edges. We see that this actually is a better description of a convex lens than a concave lens. The centre portion of a convex lens, shown right here, is its thickest part. and then, at its edges, its top and bottom its thinnest. We see then that it’s a convex and not a concave lens that decreases in thickness from its centre to its edges.

For our final answer then, we’ll choose option C. A concave lens increases in thickness from its centre to its edges.

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