# Question Video: Identifying the Optical Axis of a Concave Lens Science

The diagram shows a concave lens. Light rays pass through the lens in the horizontal direction. Which line shows the optical axis of the lens?

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### Video Transcript

The diagram shows a concave lens. Light rays pass through the lens in the horizontal direction. Which line shows the optical axis of the lens?

In this question, we are given a diagram that shows us a concave lens with five different lines passing through it. We are asked to find which line shows the optical axis of this lens.

First, let’s recall what we know about the shape of concave lenses and how they interact with light rays. A concave lens is a lens with a curved surface that is thickest along the edges and thinnest at the middle of the lens. We can create this shape by taking a cylinder and orienting its circular faces along the horizontal axis. If we then draw two spheres along the side of the cylinder, like we’ve shown here, and remove the overlapped material from the cylinder, we are left with a shape that makes a concave lens.

Next, to help us determine the optical axis, let’s take a look at the center of these spheres here. These two points in the center are called the centers of curvature because they are an equal distance from every point along the edge of their respective spheres.

If we draw lines from the centers of curvature to the edge of each sphere, then we have found our radius of curvature for these spheres. The radius of curvature will be the same length, no matter which point on the edge it is pointing towards, since the distance from the center of curvature is the same at all points.

Notice that these centers of curvature line up with the center of our lens. In fact, if we connect these points, we get a line that will go through the geometric center of this lens. We call this line the optical axis of our lens, which is what our question is asking us to find in the given diagram.

Let’s compare our two images. We are looking for the optical axis of this lens, which is the line that passes horizontally through the center of the lens. We can see that all five lines are passing through the center of our lens. But most of them are traveling through at an angle. Only line number four passes through the center and is also horizontal, which means this is our optical axis. Therefore, line four is the correct answer.