# Why Light isn't like an Acoustic wave?

I just wanted to know why light isn't an Acoustic wave.Is it because light wave doesn't obey acoustic properties?

• Light speed is orders of magnitude larger than sound speed. Also light doesn't need a medium to propagate, while acoustic wave does. Commented Nov 21, 2014 at 16:43
• Light and sound are different things even though they are both waves. Asking why light is not sound even though they are both waves is like asking why dogs are not cats even though they are both animals. If I've misunderstood, could you clarify your question? Commented Nov 21, 2014 at 16:52
• This is a legitimate and non-trivial question if you add the "like" between "isn't" and "an" in the title; after all even JC Maxwell tried to create mechanical models that emulate the propagation of EM waves as if it took place in a solid. Commented Nov 21, 2014 at 17:37
• Why is a duck not an octave? Commented Nov 21, 2014 at 21:46
• @ThePhoton:I couldn't get the meaning of octave eventhough you meant to say that light and sound can't be same. Commented Nov 22, 2014 at 4:12

• Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels.

• Both the speed of sound1 and the speed of light have fixed values regardless of the speed of their source.

• Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal.

• You can measure acoustic waves with things faster than the acoustic wave (e.g. bounce light off the wave to find its speed), you cannot do this with light (as nothing can travel faster than light and you can't bounce light off light)

• The acoustic wave we are most familiar with is a sound wave in air (e.g. speaking). This air as a medium is a lot of vibrating (moving) gas molecules, the wave is a compression/expansion wave travelling through this medium. The air is a macro system, meaning it has a lot of complexities that can effect the travel of the wave, e.g. wind direction, temperature, density, composition of gas to name a few. As mentioned light has no known medium that it travels through so is a much simpler system in comparison.

• The laws of Relativity are based on the fact that the speed of light is the same no matter the frame of reference it is measured from. This leads to a lot of strange rules, e.g. time dilation, length contraction, etc. The speed of sound is also fixed so it is interesting to consider similarities here (if you can create an equivalent world to compare, e.g. you can't measure sound waves with anything faster than sound waves and you can't detect the medium in which the sound wave travels)

In summary light seems to be a fundamental feature of the universe compared to sound or acoustic waves which are macro level features. E.g. sound waves travel through a medium made of smaller particles that follow rules more fundamental to the wave itself. You can consider that waves can form at larger scales as well, e.g. earthquakes, ripples in Saturn's rings, etc. I like to think of this as the fractal nature of the universe, e.g. features (waves) repeat themselves at different scales (with variations at each scale).

1. The speed of sound is fixed for a given uniform medium but can vary for different mediums or different properties of that medium (e.g. pressure, temperature, etc)

Acoustic Wave is a wave in which motion of one atom causes motion of another atom because it is lying next to it. Light is change in electric or magnetic field which further causes changing fields.

Acoustic waves are longitudinal waves. Light is a transversal wave, hence not an acoustic wave.

• acoustic waves are longitudinal in gases but in solids there is transversal motion, as well. Commented Nov 21, 2014 at 17:34
• But this is only true for free space. Light in a confined space, e.g. a waveguide, is not necessarily purely transverse! In fact, there are situations where TEM waves don't even exist. Commented Nov 21, 2014 at 21:20
• What about surface acoustic waves and shear waves - both have transverse components ? Commented Nov 27, 2014 at 5:35

Acoustic waves need a medium through which to travel. Light does not.