Question Video: Comparing the Densities of Media That Light Waves Travel in Physics • 9th Grade

The diagram shows the wave fronts of four identical light waves traveling through water and then passing into four other materials. Which material is densest? Which material has a density most similar to the density of water? Which material has a lower density than water?

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Video Transcript

The diagram shows the wave fronts of four identical light waves traveling through water and then passing into four other materials. Which material is densest? Which material has a density most similar to the density of water? Which material has a lower density than water?

In our diagram, we see the wave fronts of these light waves represented by a series of parallel lines. Each of the four light waves travel through water and then interacts with one of four materials. The way a given material affects this light wave can be seen by how far apart the wave fronts are spaced inside that material. We see that for some materials, the wave fronts are more compressed, whereas for others, they’re more spread apart.

In the first part of our question, we want to identify which of these four materials (A) (B), (C), or (D) is densest. To help us figure that out, let’s recall that as a wave travels into a more dense medium, its wavelength decreases. On our diagram, a decreased wavelength would appear as wave fronts that are spaced more closely together. Therefore, the material in which the wave fronts are spaced most closely together is the most dense material. Comparing the spacings between wave fronts in (A), (B), (C), and (D), we see that successive wave fronts are separated by the smallest distance in material (B). Therefore, material (B) is densest.

Moving on, we want to now figure out which material has a density most similar to the density of water. This will be the material of the four where the wave front separation in that material is most similar to the wave front separation in water. Although none of the materials seem to have an exactly equal wave front separation as the separation of waves and water, material (C) looks to be the very closest. Because the distance between successive wavelengths changes least as the wave passes from water into material (C) of all the materials, we know that material (C) has a density most similar to the density of water.

Lastly, let’s consider which material has a lower density than water. If as a wave travels into a more dense medium, its wavelength decreases, then by the same token, as a wave travels into a less dense medium, its wavelength will increase. That increase in wavelength will mean increased spacing between successive wave fronts. The one material where successive wave fronts are clearly separated by a larger distance than the wave front separation distance in water is material (D). This then is our answer to the last part of our question. The material with a lower density than water is material (D).