Video Transcript
The paths traveled by three waves are shown in the diagram. The waves travel in different regions, A and B. Two of the waves travel in region A, and one of the waves travels in region B. Regions A and B contain media that have different properties. Wave II is reflected at the boundary between the regions, and wave III is refracted at the boundary between the regions. All the waves have the same type of wave motion. Which of the following types of wave motion could wave II have? (A) Either transverse or longitudinal, (B) transverse only, or (C) longitudinal only. All the waves have the same type of wave motion. Which of the following types of wave motion could wave III have? (A) Either transverse or longitudinal, (B) transverse only, or (C) longitudinal only.
In this question, we want to determine whether waves II and III could be transverse, longitudinal, or either. Remember that, as shown in the diagram, wave II reflects at the boundary and wave III transmits or passes through and refracts at the boundary. Basically, this question is just asking what types of wave motion allow for a wave to reflect or refract.
Let’s clear some room on screen. And we’ll answer the first part of the question by thinking about reflection. We might be most familiar with reflection occurring in light rays. Recall that visible light, like all forms of electromagnetic radiation, is a transverse wave. And, for example, we often see light reflect off a smooth surface of water, creating an inverted image of the surrounding landscape. Because light can reflect, we know that transverse waves are indeed able to do so. Therefore, we can eliminate option (C) because it excludes transverse waves.
The question is, now, can longitudinal waves reflect also? The answer might not be as immediately clear as with transverse waves. But yes, longitudinal waves can and do reflect. One of the most recognizable longitudinal waves we encounter in everyday life is sound waves, which travel by means of alternating regions of air compression. Think about echo. We can yell into a canyon and moments later, apparently, hear our voice yelling back at us. Echo is simply the reflection of sound waves, which we know are longitudinal waves. Thus, we should eliminate answer choice (B) because it excludes longitudinal waves.
We’ve seen how both transverse and longitudinal waves can reflect at a boundary, like wave II does here. Therefore, answer choice (A) is correct. Wave II could have either transverse or longitudinal wave motion.
Moving on to the next part of the question, we need to think about refraction, which is when a wave changes direction as it passes into a new medium, like wave III does. Let’s once again use light rays as an example of a transverse wave. Light refracts as it passes between air and water. And we can see this in action if we stick a pencil in a glass of water and notice how the pencil appears to be disjointed or bent. This means that transverse waves do experience refraction. So let’s eliminate option (C).
Finally, we need to consider whether longitudinal waves can refract. Let’s recall that refraction is essentially caused by a wave changing speed, and in turn wavelength, when it transmits through a boundary at an angle. Whenever a wave refracts, we know it changes speed. So, does a longitudinal wave travel at different speeds in different media?
The answer is yes, it does. For example, most people are familiar with the term the speed of sound. But this typically only refers to the speed of sound in air. Sound actually travels at different speeds depending on the medium it’s traveling through. In air, sound travels at around 340 meters per second. But in water, sound travels about three times faster, at almost 1500 meters per second. In solid steel, it’s four times faster than that even, at nearly 6000 meters per second. Thus, if a sound wave approaches and passes through a medium boundary at an angle, like in the diagram, it will refract, like wave III does. It just might not be as obvious to us because we can’t see sound waves and also because they tend to spread out over an area. This means we can eliminate option (B), and we know that (A) is correct. Wave III could have either transverse or longitudinal wave motion.
We’ve seen how, whether a wave is transverse or longitudinal, both reflection and refraction are typical wave behaviors. Therefore, we know that waves II and III are the same type of wave and could either be transverse or longitudinal.
