Refraction of Sound and Waves (Why Tsunamis are Dangerous at Capes)
Oto-kun
At the seaside campsite I visited during summer vacation, there was a warning that said, "The cape is dangerous because tsunami waves gather and become higher, so do not approach when tsunami information is issued." Since the reason was not explained, everyone was wondering why the cape was dangerous.
father
Haven't I taught Oto about the refraction of waves yet?
Oto-kun
Refraction? I learned about light refraction through lenses in school, but is there any relationship between wave refraction and tsunamis becoming higher at capes?
father
Ah, it is related. Then, let me explain how refraction occurs in the case of sound, which we always talk about.
Oto-kun
Does sound also refract?
father
Yes. It's quite difficult to experience the phenomenon of refraction in the case of sound.
Sound travels not only through air but also through water, as you know from experience when you swim in a pool and can hear sounds underwater.
Sound travels not only through air but also through water, as you know from experience when you swim in a pool and can hear sounds underwater.
Oto-kun
Earlier, I heard from Dad that the speed of sound in water is several times faster than in air.
father
Yeah. That's the point.
Oto-kun
Huh? Is there a relationship between sound refraction and the difference in the speed of sound in air and water?
father
Yes. When sound enters water from air or when sound in water comes out into air, a refraction phenomenon occurs, and this happens because the speed of sound propagating through the media is different.
Oto-kun
Hmm. I don't really understand, but.
father
I will draw a figure (Figure 1) and explain it.
First, let's consider the case where sound enters from the air into the water surface, refracts there, and propagates through the water.
Sound is a spherical wave that spreads out in a spherical shape from the sound source (wave source), but when the sound source is far enough away or when considering a sufficiently small part of the wavefront, the wavefront can be considered as a plane (plane wave).
This diagram shows how the wave propagates from point A, where the sound reaches the water surface, to point C, where point B, which was on the same wavefront as A, reaches the water surface. For example, if the propagation speed in water is the same as in air, point A would transmit to point D, and refraction would not occur. In reality, the speed of sound propagation in water is faster than in air, so it would travel a long distance while moving from point B to point C. And since it spreads as a spherical wave and has a relationship of circles and tangents, it would transmit to point E as shown in the diagram.
First, let's consider the case where sound enters from the air into the water surface, refracts there, and propagates through the water.
Sound is a spherical wave that spreads out in a spherical shape from the sound source (wave source), but when the sound source is far enough away or when considering a sufficiently small part of the wavefront, the wavefront can be considered as a plane (plane wave).
This diagram shows how the wave propagates from point A, where the sound reaches the water surface, to point C, where point B, which was on the same wavefront as A, reaches the water surface. For example, if the propagation speed in water is the same as in air, point A would transmit to point D, and refraction would not occur. In reality, the speed of sound propagation in water is faster than in air, so it would travel a long distance while moving from point B to point C. And since it spreads as a spherical wave and has a relationship of circles and tangents, it would transmit to point E as shown in the diagram.
Oto-kun
Is it the same for light?
father
That's right. When you put a stick in water, it often looks bent, doesn't it? Since both light and sound are waves, the phenomenon of refraction occurs in the same way.
Oto-kun
Hmm. I somewhat understand that the difference in the speed of waves transmitted through different media causes refraction. But how is the gathering and heightening of waves at a cape related to wave refraction?
father
That's right. Just as refraction occurs in air when there are significant temperature differences affecting the speed, refraction also occurs in water when the speed of waves changes.
Oto-kun
What does it mean that the speed of waves changes as they move through water?
father
The propagation speed of tsunami waves is actually influenced by the depth of the seabed. The shallower the water depth, the slower the propagation speed of the tsunami.
Oto-kun
I have come to understand somewhat. It seems that the cape and the depth of the sea are related.
father
Just like contour lines on land, lines that connect points of equal depth from the water surface to the seabed are called isobaths. When considering the isobaths around a cape, if the terrain is shaped like a tongue protruding as shown in this diagram (Figure 2), then at the position of line A drawn horizontally, the water depth will be shallower as it approaches the tip of the cape. The speed of the waves there will decrease, causing the refracted waves to converge.
Oto-kun
I see, I understand well. I will explain it to everyone in the class.
...I wonder if the teacher knows!?
...I wonder if the teacher knows!?