# Light versus sound waves : velocity

1. Aug 1, 2011

### gkangelexa

I noticed that in water, sound travels faster than in air, but when light hits water, its velocity decreases.

Is this because sound is a longitudinal wave and light is a transverse (electromagnetic) wave??

2. Aug 1, 2011

### xts

No. It has nothing to it.
Why do you think so?

Last edited: Aug 1, 2011
3. Aug 1, 2011

### gkangelexa

what do you mean it has nothing to it :-(?

4. Aug 1, 2011

### xts

That there is no causal dependence in (any direction) between longitudinal/transversal character of the waves and direction of the change of wave speed between air and water.

I am just curious: why did you think they should be related?

5. Aug 1, 2011

### y33t

Wave propagation speed is related to structure of the medium and wave frequency. Each medium responds differently to different frequency ranges. Sound travels faster in water because it is transferred by sonic vibrations through medium content. Water is denser than air thus it will be more efficient for transmission of sound waves. Another reason to this is that low frequency signals have more penetration depth than than high frequency signals.

When light enters water medium, water molecules should align to such a polarization that light will be able to penetrate through. Depending on the molecular structure there will be an extra delay for wave to propagate which is originating from the lag to set medium to a polarization state. Integrate this lag through the transmission path and you'll see that the measured amount of distance is traveled less than velocity 'c'. Relative phase velocity of light can be extracted from refractive index information which can be calculated from Fresnel Equations.

6. Aug 1, 2011

### xts

Would it be different, if they had to align longitudinally rather than lateraly?

Really? So why speed of sound in Helium is higher than in air? Density of Helium is much lower than of air.

7. Aug 1, 2011

### Subductionzon

The speed of sound in air is directly related to the speed of the air molecules. I hope you have heard of the Ideal Gas Law. It is PV = nRT where P is the pressure of the gas, V is the volume, n is the number of moles or the amount of gas, R is the gas constant and T is the temperature in absolute degrees.

If you notice the mass of the gas in question does not enter the equation. Since pressure is a result of elastic collisions between the air molecules and the container that implies that the lighter a gas is the faster the molecules are moving. Helium atoms are very light, they have roughly one seventh of the mass of the average air molecule. To transfer the same energy in a collision they have to be moving 2.7 times as fast as an average air molecule. Now the speed of sound is a bit more complicated than that, there is a correction factor involving the adiabatic constant of the gas in question. But the velocity needed for one collision is a good starting point to understanding why the velocity of sound is higher in helium.

For an article that gives you the actual formulas for the speed of sound in a gas you can go here:http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3.html" [Broken]

Last edited by a moderator: May 5, 2017
8. Aug 1, 2011

### y33t

There is no linear bridge between wave propagation characteristics of different phases of a medium. From penetration depth approach density can be taken as reference for comparison. As stated by others comparison between same phases of different mediums (gas to gas) can be done and molecular mass is at the denominator of the expression. Fresnel equations can be applied between any medium at any phase.

9. Aug 1, 2011

### xts

So your arguments, which you gave to OP question (you must see he has pretty poor background and really easily gets confused by unclear argumentation) were wrong and misleading.

Again:
You gave higher density of water than air as an explanation of higher sound velocity.

You gave this argument in the context, where OP suspects if longitudinal/transversal nature of the wave affects wave speed. I just (rethorically) asked if you may defend this argument, as it applies equally to longitudinal and transversal waves (despite there is longitudinal light...).

Last edited: Aug 1, 2011
10. Aug 1, 2011

### y33t

It might be misleading but this doesn't give you the right to say 'were wrong'. Density is a factor in penetration depth, thus affects the wave speed on medium, period. Polarization density is related to wave speed in medium through constitutive relations. Since air is composed of different materials with different ratios it is not meaningful to compare different phases of these mediums refractive indexes. Light will move faster in any gas than any liquid. It is a pedological analogy to a person at this level of interest.

Light can have any polarization type and it will affect it's speed in the medium. But this is just one of the reasons why sound travels faster than in air and light's velocity decreases when it enters from air to water. One of the contributing phenomenas.

11. Aug 2, 2011

### harrylin

No, that has nothing to do with it. Water and air are the media of sound vibration while they are mere obstacles for light propagation. Without atoms there is no sound propagation while light propagation proceeds unhindered.

It may be that there is a kind of relationship, in the sense that perhaps a more dense medium usually has a higher speed of sound while it reduces more the speed of light (I did not check this).

Best,
Harald

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