Video: Convex Lenses

Which of the objects shown in the diagram is a convex lens? [A] Graph A [B] Graph B [C] Graph C [D] Graph D [E] Graph E.

02:20

Video Transcript

Which of the objects shown in the diagram is a convex lens?

And we’ve been given options A to E from which we need to choose. So first of all, what is a convex lens? Well, convex refers to a lens that’s thicker at the centre than at the edges. Now, it’s important to remember that the diagrams A to E show cross sections of lenses. In other words, if we’ve got a lens here which we look through, then the diagrams A to E were made by first slicing the lens in half and then looking along this direction. So if we put our eyeball here, we’re looking in this direction. Therefore, the edges of the lens which is this part here corresponds to the top and bottom of each diagram. And the centre of the lens corresponds to this region here.

So now, for a convex lens, we need a lens which is thicker at the centre than at the edges, which means we can immediately rule out lens A, which is the same thickness all the way along, as well as lens B, which gets thinner at the centre than at the edges, and lens C which is the same. What we’re left with are lenses D and E.

Now, both of these are thicker at the centre than they are at the edges. So which one is the convex lens? Well, technically, they’re both convex. But when we say convex lens, we’re often referring to what’s known as a biconvex lens. And this is a lens which is curved on both sides. In other words, it’s convex on the left and on the right. And this lens looks like option E.

So what’s going on with option D then? Well, lens D is what’s known as when it comes to lens D, we’ve got what’s known as a plano-convex lens: one side is the plane — the flat — surface and the other is the convex part. Now, as we’ve mentioned earlier, when we say a convex lens, this most commonly refers to what we call in technical terms a biconvex lens. And if we want to describe lens D, then we would call it a plano-convex lens.

Hence, we’re not looking for lens D as the answer to our question. And so our final answer is that lens E is a convex lens.

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