# Question Video: Determining the Path of a Ray That Passes through the Center of a Concave Lens and Is Parallel to the Optical Axis Science

Each of the following diagrams shows a ray entering a thin concave lens. The point marked P is the focal point of the lens. Before the ray enters the lens, it is parallel to the optical axis. It passes through the center of the lens. Which diagram correctly shows the path of the ray after it passes through the lens? [A] Diagram A [B] Diagram B [C] Diagram C

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### Video Transcript

Each of the following diagrams shows a ray entering a thin concave lens. The point marked P is the focal point of the lens. Before the ray enters the lens, it is parallel to the optical axis. It passes through the center of the lens. Which diagram correctly shows the path of the ray after it passes through the lens? Diagram (A), diagram (B), or diagram (C).

This question is asking us about a light ray that is initially parallel to the optical axis and passes through the center of the lens. In order to answer this question, we must recall a rule for light rays passing through a concave lens. The rule states that any light ray that passes through the center of a concave lens will exit the lens traveling in the same direction as when it was incident on the lens.

The problem states that the ray of light passes through the center of the concave lens. So we know that the ray of light will not change direction after passing through the lens. Now we just need to look at each of these options and identify which ray diagram correctly shows the ray traveling along the same direction after passing through the lens.

First, let’s look at option (A). If we trace the direction of the light ray exiting the lens backward through the lens without changing direction, as shown by the blue dashed line, then we can see that it does not match the direction of the ray as it enters the lens. Therefore, we know that this cannot by the correct option because the ray exits the lens traveling in a different direction then that it traveled in before entering the lens. This means we can eliminate answer choice (A).

Next, let’s look at option (C). Again, we can trace the direction of the light ray after exiting the lens backward through the center of the lens. And again, we see that the trace does not match the direction of the ray entering the lens. So option (C) cannot be the correct option either because the ray exits the lens traveling in a different direction to that it traveled in before entering the lens. We can therefore eliminate answer choice (C).

That leaves us with option (B). Let’s again trace back the direction of the light ray after traveling through the center of the lens to verify that this is the correct answer. When tracing this exiting ray back through the center of the lens, it matches perfectly with the ray of light entering the lens. This ray exits the lens traveling in the same direction as that it traveled in before entering the lens. So it follows the rule for light rays passing through the center of a concave lens. And so, we have confirmed that option (B) is the correct answer.

The diagram which correctly shows the path of the light ray after it passes through the concave lens is diagram (B).