Video Transcript
Which of the following is a convex lens? (A), (B), (C), or (D).
Here, we want to identify the correct shape of a convex lens. To think about this, let’s quickly recall that a lens is used to focus rays of light
at a point and that there are different types of lenses depending on exactly how we
want to focus the light.
A convex lens must be thicker at the middle and thinner at the edge. If we look at answer choice (A), we see that it’s thinner at the middle and thicker
at the edge. Knowing this, we can eliminate it. We can also look at answer choice (B), it’s not thicker in the middle and thinner at
the edge either. So we can eliminate answer choice (B) as well.
Now the hexagonal shape shown in option (D) is thicker at the middle and thinner at
the edge, but notice that it has straight edges that form angles. A proper lens should have curved sides. Therefore, we know answer choice (D) is incorrect as well.
Now when we look at a lens from the side, we should see arced curves. Specifically, a convex lens should look like the overlap between two circles. Let’s draw a diagram to show what this looks like. Notice that the area shaded in blue here has curved sides. And it’s thicker at the middle and thinner at the edges. This is the correct shape of a convex lens. And it looks just like the lens in answer choice (C). This is a convex lens, and thus we know that (C) is the correct answer.
