Sound wave vs light wave speed

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Discussion Overview

The discussion revolves around the speed of sound and light waves in different media, specifically comparing sound waves in incompressible materials like water to light waves in less dense materials like air. Participants explore the relationship between wave types (longitudinal vs. transverse) and their propagation speeds in various contexts, including elasticity and density.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that sound waves are mechanical and require a medium to propagate, while light waves are oscillations of electric and magnetic fields.
  • It is proposed that the speed of sound depends on elasticity rather than density, with higher speeds in more elastic materials.
  • One participant suggests that sound waves might travel faster in less elastic media, which is challenged by others who argue that elasticity is crucial for sound speed.
  • There is a discussion about how light waves interact with media, with some stating that denser media hinder light propagation due to molecular interactions.
  • A participant raises a speculative question about the nature of light and its generation, suggesting that it may not solely result from classical electron valance changes.
  • Another participant expresses skepticism about the speculative nature of the question regarding light, indicating it may not align with established physics discussions.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between elasticity, density, and wave speed, indicating that there is no consensus on these points. The speculative nature of the discussion about light generation also highlights a lack of agreement on that topic.

Contextual Notes

Some claims about the speed of sound and light depend on specific definitions and assumptions about elasticity and density, which remain unresolved in the discussion.

silversurf
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Why do sound waves travel faster in incompressible material (such as water (water vs air)) and why do light waves travel faster in less dense (such as air (air vs water))?

I know that sound waves are longitudinal waves and light waves are transverse waves. What is the relationship between longitudinal/transverse waves with regard to speed in water vs air?
 
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Light and sound are totally different things. Sound waves are mechanical in nature, vibrating molecules bumping into each other. Light is an oscillation of electric and magnetic fields.
 
Ah I see. Would you happen to know why sound waves travel faster in more dense material and why light waves travel faster in less dense?
 
The speed of sound depends on elasticity (basically, rigidity), not density. It is higher for more elastic materials, not denser materials. So for example, it is higher in aluminum than in lead.
http://www.elcometer.com/en/component/productmanager/productmanager?prod=657

For light, electric and magnetic fields do not transmit through media as well as through a vacuum. http://en.wikipedia.org/wiki/Permittivity
 
An interesting point to make with regards to this is that for a sound wave to propagate, there is a medium that must permit the energy to flow through it.

With regards to elasticity, intuition would suggest that it is reasonable for sound to travel faster in a less elastic medium. When a ball is bounced on concrete, the concrete absorbs less energy than foam would. A less elastic medium wants to return to its equilibrium position quickly. A very elastic medium takes its time, which suggests that the wave should travel more slowly through it.

As for an EM wave, the necessary details lie in how the medium transmits the wave. I understand it to be absorption and emission of the energy from the EM wave by the matter, which takes time. So less dense is closer to a vacuum which is faster.
 
geologic said:
With regards to elasticity, intuition would suggest that it is reasonable for sound to travel faster in a less elastic medium. When a ball is bounced on concrete, the concrete absorbs less energy than foam would. A less elastic medium wants to return to its equilibrium position quickly. A very elastic medium takes its time, which suggests that the wave should travel more slowly through it.
You have the concept of elasticity backwards.
 
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russ_watters said:
The speed of sound depends on elasticity (basically, rigidity), not density. It is higher for more elastic materials, not denser materials. So for example, it is higher in aluminum than in lead.
http://www.elcometer.com/en/component/productmanager/productmanager?prod=657

For light, electric and magnetic fields do not transmit through media as well as through a vacuum. http://en.wikipedia.org/wiki/Permittivity

With respect, the formula for speed includes both factors.
From Wiki and many other sources:
In general, the speed of sound c is given by the Newton-Laplace equation:

c = √(K/ρ)

where

K is a coefficient of stiffness, the bulk modulus (or the modulus of bulk elasticity for gases),
ρ is the density

If you consider a very simple, one dimensional, model of a row of masses linked together by springs, it is (almost) intuitive that the rate of propagation of a disturbance will be higher for stiffer springs and lower for greater masses.
 
silversurf said:
Why do sound waves travel faster in incompressible material (such as water (water vs air)) and why do light waves travel faster in less dense (such as air (air vs water))?

I know that sound waves are longitudinal waves and light waves are transverse waves. What is the relationship between longitudinal/transverse waves with regard to speed in water vs air?
Adding a bit to earlier comments:
Sound waves need the water (or other molecules) to propagate: they are the vibration of those molecules. In contrast, the water hinders (retards) the propagation of light waves as the molecules interact with those waves.

For more on light propagation see for example this thread:
https://www.physicsforums.com/showthread.php?t=613481
 
  • #10
I have an additional question related to this subject. Has anyone considered that illumination commonly referred to as light may be a property of a physical reaction. And, that light may not always be resulting from the classical electron valance change. There might be other physical phenomena that generate illumination. This would turn astronomy upside down. Is this stupid? this has been bugging me for a while.
 
  • #11
jafo1 said:
I have an additional question related to this subject. Has anyone considered that illumination commonly referred to as light may be a property of a physical reaction. And, that light may not always be resulting from the classical electron valance change. There might be other physical phenomena that generate illumination. This would turn astronomy upside down. Is this stupid? this has been bugging me for a while.

I wouldn't use that word. 'Unfounded' would be a more polite word. This is all much too speculative for PF, I think and definitely not Physics.
 

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