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Question Video: Identifying Refracted Wave Fronts Physics • 9th Grade

The diagram shows the wave fronts of a light wave traveling in air that is incident on the surface of a glass block. Which of the other diagrams best shows the positions and sizes of the wave fronts as the wave travels in the glass block? [A] Diagram A [B] Diagram B [C] Diagram C [D] Diagram D [E] Diagram E

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Video Transcript

The diagram shows the wave fronts of a light wave traveling in air that is incident on the surface of a glass block. Which of the other diagrams best shows the positions and sizes of the wave fronts as the wave travels in the glass block? Our answer options are diagrams (A), (B), (C), (D), and (E).

This is a question about light refracting as it travels from one medium to another. Remember that refraction is the change in direction of light. Light travels at different speeds through media of different densities and changes direction at a boundary between media of different densities. Specifically, light travels more slowly through a medium with higher density.

Looking at the diagrams in the question, we can see light entering a glass block from air. In this diagram, the light incident on the glass block is drawn as a series of wave fronts. A wave front represents a peak in a transverse wave, as if we were looking at the wave from above. The wave itself travels in a direction perpendicular to the wave fronts.

We know that the density of air is less than the density of the glass block. So we also know that the speed of light in the glass block will be less than that in the air. Each wave front in this first diagram is the same distance apart. This distance is the wavelength of the light. We can zoom in on this light and show the wavelength. As the wave passes through to a different medium, its frequency will remain the same. This is because the frequency determines the energy of the light. And just passing through to a different medium will not change the energy of this wave.

However, we have seen that the speed of the wave changes as it passes through to the new medium. At this point, we can recall the equation that relates the speed of a wave, its frequency, and its wavelength. 𝑣 is the speed of the wave, 𝑓 is the frequency, and 𝜆 is the wavelength. Let’s say that in air, the speed of the wave is 𝑣 sub air, the wavelength is 𝜆 sub air, and the frequency is 𝑓 sub air.

When the light wave passes into glass, we know the speed decreases, so 𝑣 sub glass is less than 𝑣 sub air. The frequency is still the same. 𝑓 sub glass equals 𝑓 sub air. And so for this equation to hold, the wavelength must also decrease. 𝜆 sub glass is less than 𝜆 sub air. In other words, when the light wave passes from air to glass, we should see its wavelength decrease. And so the wave fronts will get closer together. This immediately eliminates answer option (D), as here the wavelength does not decrease.

We also mentioned earlier that when light passes into a medium with a different density, it will change direction. This allows us to eliminate answer option (B). In this answer option, the wave is not changing direction.

In answer options (C) and (E), the wave appears to be getting wider in some way. This would not happen when a wave passes into a medium of different density. The only thing that should happen is that the wave fronts change direction. Hence, we can eliminate answer options (C) and (E).

And finally, we see that answer option (A) shows us the expected behavior. As the wave passes into a more dense medium, the direction in which it is traveling changes, and the wavelength decreases. So the correct answer is option (A).

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