# Question Video: Determining the Paths of Rays Passing through a Convex Lens Science

The diagram shows five light rays that will pass through a thin convex lens. Which of the light rays will not change direction as it passes through the lens?

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### Video Transcript

The diagram shows five light rays that will pass through a thin convex lens. Which of the light rays will not change direction as it passes through the lens?

Here, we are given a diagram of a convex lens with five rays of light that will pass through it. We are asked to figure out which ray will not change direction when it passes through the lens. Let’s begin by refreshing our memories about what a convex lens is and how light rays behave when passing through them.

First, let’s look at the shape of a convex lens. As viewed from the side, it can be created by overlapping two circles and taking the section in the middle. We can see that our lens is thinnest along the edges, while it is thickest in the middle of the lens. This shape allows convex lenses to focus light rays as they pass through and make them converge at a point on the other side of the lens. Light rays that are parallel to the optical axis but not on it will have their directions changed and be bent towards the optical axis and converge at a point called the focal point.

Now, let’s take a look at the center of the lens. And notice that it is aligned with the center of curvature of each of the circles that determine the shape of the lens. A center of curvature of a circle is a point that is at equal distance from every point on the circumference of a circle. This distance is called the radius of curvature, and it equals the radius of the circle.

If we connect the centers of curvature with a line, this line passes through the center of the lens and it defines the optical axis of the lens. If we take the point on the optical axis that is at the center of the lens, it is a special point in a convex lens. If any light rays pass through this point, they will pass through without changing direction. Knowing this, we can look at the diagram in the question and find the ray that passes through the point along the optical axis at the center of the lens.

Looking at the five rays we are given, ray one and ray two are above the optical axis, while ray four and ray five are below the optical axis. These rays all travel horizontally parallel to the optical axis, which means they cannot pass through the center of the convex lens and instead will be focused and pass through the focal point on the other side of the lens to the side that they entered the lens from. Ray three travels along the optical axis and so will pass through the center of the lens. Ray three then will not change direction while passing through the lens. Ray three is the correct answer.