Question Video: Identifying Refraction of a Light Ray | Nagwa Question Video: Identifying Refraction of a Light Ray | Nagwa

Question Video: Identifying Refraction of a Light Ray Physics • Second Year of Secondary School

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Which of the diagrams correctly shows refraction of a light ray, assuming that the substances the light ray moves through have different densities? [A] Diagram a [B] Diagram b [C] Diagram c [D] Diagram d

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Video Transcript

Which of the diagrams correctly shows refraction of a light ray, assuming that the substances the light ray moves through have different densities?

This is a question about light entering and then leaving a substance that has a different density to the medium it was originally traveling through. To answer this question, we need to identify the diagram that shows the light ray emerging from this gray block in the correct direction. We’re not shown the path of the light ray inside the block. So we’ll have to draw it in ourselves and use our knowledge of refraction to recognize which diagram shows a valid path for the light ray.

Remember that refraction is the change of direction of light as it passes from one medium to another. This occurs because light travels at different speeds in different media. In all four diagrams, the light passes between different media two times and thus should refract two times. The first refraction occurs when light enters the gray block. The second refraction occurs when it exits the gray block. So we know that the light should change its direction of travel twice. This means we can immediately eliminate diagram (b).

As we can see, by connecting the points where the light ray leaves and enters the block, the light doesn’t change direction either when it enters the new medium of the block or when it leaves the block. This could only be the case if the substances the light travels through have the same densities. But we were told that the substances do have different densities. So, diagram (b) does not correctly show refraction.

Now, we know that when light is refracted, it is bent either towards or away from the surface normal. Remember that the normal is simply the line perpendicular to the surface of the medium boundary. When light enters a less dense medium and speeds up, it is bent towards the normal. When light enters a more dense medium and slows down, it is bent away from the normal. Notice that in both cases, light transmits or passes through the medium boundary and thus crosses the surface normal at the point where it enters the new medium.

But now take a look at this example diagram, and notice that here the light is shown as transmitting or passing into the new medium. But it almost seems to be reflecting from the surface normal at the point where it enters the new medium. This is not how light refracts. In reality, when light transmits and refracts, the refracted ray should move through the new medium on the opposite side of the surface normal as the original, incident ray.

It’s only possible for the refracted ray to be bent somewhere within this 90-degree range, between the surface boundary and its normal, but on the opposite side of the normal as the incident ray. It’s not possible for this ray to be refracted so that it travels over here, on the same side of the normal as the incident ray. Keeping this in mind, we can eliminate diagram (c).

To better understand why (c) is wrong, let’s draw the path that the light ray takes inside the block. We can see that at both medium boundaries, the light ray’s path violates these principles we just discussed. This supposed path strangely seems to be somehow reflected from the surface normal both times. But we know this can’t be right, so diagram (c) is incorrect.

Note that, in the same way, we can also deduce that diagram (d) is incorrect. Here, the light ray seems to behave at the first medium boundary, where it enters the block. But then when the light leaves the block, it’s not possible for it to actually take this path, since it should transmit on the opposite side of the surface normal. Thus, diagram (d) is incorrect.

That leaves us with (a), so let’s take a closer look at this diagram. As the light enters the block, it’s bent toward the surface normal. This tells us the ray must be moving from a lower-density medium, the space around the block, to a higher-density medium, the gray block. When the light ray leaves the block, it is refracted a second time and bent away from the normal, which is what we expect, since here the ray is moving back to a lower-density medium.

So diagram (a) represents a perfectly sensible representation of refraction for these media. Therefore, we know that diagram (a) shows the correct path of a light ray moving through media of different densities.

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