speed of a wave air versus water

[MedGrl@2022]

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I am confused about the velocity of waves.

In chapter 6 of my examkracker physics book I learned that waves move faster in water than air because although water has a greater density than air... it has a greater bulk modulus and v=(sqrt(B/p)). Then in chapter 8, we know that water has a great index of index of refraction than air therefore as light passes from air to water the frequency stays the same but the wavelength becomes shorter. This suggests that light travels slower in water than in air.

Is the example, in chapter 6 for sound waves/longitudinal waves only? EK used transverse wave examples too.

If anyone can help me understand this that would be great.

Thank you,

Verónica

 

[image187]

if you shined a flashlight into a brick wall, how fast would you see the visible light on the other side? probably not at all because it wouldn't penetrate the wall. How far can you yell in outer space? Not far at all because there is no medium for the sound waves. This is how I think... very simply. You can use these equations and all, and use snell's law to figure out the angle the light will bend when it hits the
water, but I intuitively know that it bends downwards to the vertical because the water is more dense and it slows it down from the air it was just in. If the liquid was mercury, it would slow and bend even further. As far as sound, I know that there needs to be a medium for the sound waves, and more dense things
let the sound travel more efficiently. You can probably figure out these intuitive things from equations, but on the MCAT you should seriously start trying to think as simply as possible about these things. Youll get a lot of questions right very fast that way.

 

[Gauss44]

I am confused about the velocity of waves.

In chapter 6 of my examkracker physics book I learned that waves move faster in water than air because although water has a greater density than air... it has a greater bulk modulus and v=(sqrt(B/p)). Then in chapter 8, we know that water has a great index of index of refraction than air therefore as light passes from air to water the frequency stays the same but the wavelength becomes shorter. This suggests that light travels slower in water than in air.

Is the example, in chapter 6 for sound waves/longitudinal waves only? EK used transverse wave examples too.

If anyone can help me understand this that would be great.

Thank you,

Verónica

Yeah, I think the difference is whether you're dealing with a longitudinal wave or a transverse wave.

 

[MedGrl@2022]

Yeah, I think the difference is whether you're dealing with a longitudinal wave or a transverse wave.

They used a similar example with a transverse wave travelling through a rope... and that also depended on a medium... I wonder now if it has to do with the properties of light since light is an electromagnetic wave that can travel through a vacuum (and it travels fastest through a vacuum). I think all other waves need to travel through a medium. Thus, there must be more differences, right? Also light is a particle too... thus other particles could block it. I think light just might be a special wave since it is already a particle.

Does this make sense?

 

[shoehornlettuce]

Light is electromagnetic radiation and is completely different from sound waves (not to be confused with radio waves which are also electromagnetic radiation). All electromagnetic radiation (visible light, IR, x-ray, radio etc.) travels at the speed of light in a vacuum.

Think of sound waves as a series of rarefactions and compressions of a medium. Take air for example. You'll have an area of high pressure (compression) followed by an area of low pressure (rarefaction) repeated over and over. If there was no medium (air particles) then there is nothing to compress (reason why sound can't travel through a vacuum).

Now think about the density of the medium. As the sound wave propagates through a medium there is a disturbance in the medium (areas of high and low pressure in air for example). The more dense the medium, the faster the particles that were disturbed will try to "snap back" to their original position. As they snap back, the wave moves forward through the medium. So the faster they snap back (more dense the medium) the faster the wave propagates through it.