# Sound wave vs light wave speed

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?

russ_watters
Mentor
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?

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.

russ_watters
Mentor
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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sophiecentaur
Gold Member
2020 Award
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.

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?
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:

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.

sophiecentaur