Why does sound wave travel faster in a denser medium whereas light travels slower?
Asked by: Shawn Tee
Sound is a mechanical wave and travels by compression
and rarefaction of the medium. Its velocity in an elastic medium is
proportional to the square root of Tension in the medium.
A higher density leads to more elasticity in the medium
and hence the ease by which compression and rarefaction can
take place. This way the velocity of sound increases by
increase in density.
Light on the other hand is a transverse electromagnetic wave.
It does not depend on the elastic property of the medium
in which it travels. Its velocity in a medium is determined
by the electromagnetic (e.g. dielectric) properties of the
medium. Effective path length on the other hand is increased
by an increase in the density and hence it leads to higher
refractive index and lower velocity.
Answered by: Shandar Ahmad, Ph.D. in Physics, Jamia Millia Islamia, New Delhi
This is is a good question since it forces us to interpret what is meant by a sound
wave and a light wave. This may indeed get into complex subject matter, thus I will
first give a brief answer followed by explanations.
1)Sound requires a medium in which to travel, such as a gas, liquid, or solid. This
is why there can be no sound in outer space since it is a vacuum (i.e. absent of
matter). Sound is essentially a pressure wave which causes the medium (i.e.
molecules in air) to oscillate, this oscillation is thus propagated through the
medium by the matter making it up. Hence we can argue that since a medium is
required for sound to propagate, and that the closer the molecules in that medium are
to each other the faster the sound will travel.
2)Light does not require a medium (one of the discoveries that mark the beginning of
modern physics) in which to propagate. What light is, when we think of it as a wave,
is a pair of oscillating electric and magnetic fields which propagate through space.
In the absence of matter electromagnetic waves (e.g. light) are free to move at
their maximum velocity (~3*10^8 m/s), but when traveling though matter the velocity
of light is dependent on the conductivity of the substance. This is actually a very
complicated matter, which perhaps is best if we leave it at this: since light becomes
dependent on the ability of the molecules making up the substance to move the E-M
waves from one point to another the light wave slows down (in relation to its
velocity in a vacuum).
Answered by: Adrian Soldatenko, Undergraduate at UCLA in physics
Neither answer published corrects the assumption that increased density means increased speed for sound. Actually,
increased density DECREASES the speed of sound in a medium.
While increased density can mean increased rigidity, or stiffness, it is not always the case. Greater density can be due to each molecule or atom having more momentum, and being slower to respond to the vibration of its neighbor. The formula for determining the speed of sound in any material shows an INVERSE relationship between velocity and
density, but a DIRECT relationship with the material's rigidity.
Answered by: Paul Walorski - A.B. Physics