# Question Video: Identifying How a Concave Lens Changes the Path of Parallel Rays of Light Science

Which of the following diagrams shows what happens when parallel light rays pass through a thin concave lens? [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D [E] Diagram E

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### Video Transcript

Which of the following diagrams shows what happens when parallel light rays pass through a thin concave lens? (A), (B), (C), (D), or (E).

This question is asking us to recognize the effect that concave lenses have on parallel rays of light. Before we answer this question, it’ll be useful to recap what we know about concave lenses. A concave lens looks like this. When light reaches the lens, the lens changes the path of the light. When parallel rays of light reach a concave lens, the lens causes the rays of light to spread out. Once they’ve passed through the lens, they’re no longer parallel to each other. And the distance between each ray of light increases as the distance of a light from the lens increases.

When rays of light spread out from each other, we say they are diverging. Because a concave lens causes light rays to diverge, a concave lens is sometimes called a diverging lens. If we look at these diverging light rays, we might notice something interesting. If we trace each of these rays backwards, ignoring the lens, it appears as though all of the rays come from a single point. We call this point the focal point. Note that the actual light rays don’t all physically pass through this point. It just appears that they come from this point because of the way the lens has caused the rays to spread out.

This diagram actually sums up the most important things we need to remember about concave lenses. Concave lenses cause parallel rays of light to diverge. When this happens, it appears that the rays of light are coming from a point behind the lens, called the focal point.

With this in mind, we are ready to tackle this question. Let’s start with option (E). In this diagram, it looks like the lens isn’t doing anything at all. The light rays are parallel before they enter the lens, and they’re parallel after passing through the lens. This is the same path that the light rays would take if the lens wasn’t there. So option (E) cannot be the correct option.

Next, let’s look at options (C) and (D). Option (C) shows all of the light rays bending downwards by the same amount, and option (D) shows all of the light rays bending upwards by the same amount. Although the light rays have changed direction, because each ray has changed direction by the same amount, the light rays are still parallel after passing through the lens. They’re certainly not diverging in the way that we would expect if they’d passed through a concave lens. So these cannot be correct answers.

Next, let’s think about (B). At first glance, this might seem like a good option. If we look here, we can see that the rays are spreading out, after having come from a point. However, option (B) is not the correct answer. To understand why, we have to look at what’s happening at the surface of the lens. If we look at this part of the diagram, we see that the lens is actually causing the light rays to move towards each other and not to spread out. These rays then meet at this point, in front of the lens. But this isn’t what concave lenses do either. Concave lenses cause the rays to spread out so it appears as if they came from a point behind the lens. So option (B) is not correct.

That leaves us with option (A). Option (A) shows the light rays spreading out after they pass through the lens. If we trace these rays back, it looks as though the rays have come from a point behind the lens. This is exactly what we would expect to see when parallel light rays pass through a concave lens. So option (A) is the correct answer.