# How are sound waves different than electromagnetic waves?

How are sound waves different than electromagnetic waves?
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A few differences between sound waves and Electromagnetic waves may be summarized as -
1) Sound waves are longitudinal in nature, i.e. the vibrations happen parallel to the direction of propagation of the wave. However, light waves (or other EM waves) are transverse i.e. the disturbance occurs perpendicular to the direction of propagation.
2) Sound waves are elastic waves and propagate through actual vibrations of constituent particles in the medium it travels though. The EM waves on the other hand are due to oscillating electric and magnetic fields perpendicular mutually and also to the direction of motion. There is no actual vibration of material particles needed to sustain the wave.
3) As a consequence of the previous point, sound waves need a material medium for their propagation while EM waves need not have an material medium for their propagation (they may propagate through vacuum as well and that's how light from the sun reaches the earth through the vast space which is essentially a vacuum).
4) For sound waves, the velocity of the waves depends on several mechanical properties of the material medium, density being the most important - sound waves travel faster through denser mediums. Sound travels faster through an iron rod than through air!
On the other hand, the speed of EM waves in a medium depends on the electrical and magnetic properties. The speed of light through a material medium is,
$v=\frac{1}{\sqrt{ϵ\mu }}$ where $ϵ$ and $\mu$ are electrical permittivity and magnetic permeability of the medium.
EM waves travel fastest through vacuum and their speed decreases in the following order (in general) - Gas>liquid>solid.
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