Particles and waves

[nutronjon]

Why do sound waves go around corners and not light waves?

[Thunderbird]

Excellent question, it has to do with wave length, and the medium in which it propagates in.

Sound is the disturbance of mechanical energy, and moves through matter as a wave. It is the properties of a sound wave that characterise sound namely frequency, wavelength, period, amplitude, and speed.

Light is electromagnetic radiation.

There are two main differences between sound and light waves. Firstly in speed, sound travels at about 1,100 feet per second, light waves travel at about 186,000 miles per second.

The second difference is that sound requires a solid, liquid or gaseous medium to travel but light can travel through empty space. The denser the medium, the greater the speed of sound whereas the opposite is true of light. Light travels around a third slower in water than in air, sound travels through all substances but light will not pass through anything that is opaque.

I know this is not adaquate answer, but its a start.

[Moontanman]

Actually light in an atmosphere does indeed travel around corners. That's why shadows on Earth are not absolute darkness. If you look at the moon you can see the terminator as a distinct line. If you were on the moon looking at the Earth you would see the terminator as a broad fuzzy area instead of a distinct line. Now having said that i am sure that doesn't answer your question. Light doesn't need a substance to travel through, it goes in a more or less straight line. (in the real world light can bend around corners in a limited way but that isn't really what we are talking about.) Sound on the other hand follows what ever it it is traveling through. That allows sound to bend around corners as it follows it's transmitting medium. sound travels faster in water than it does on air and faster in steel than it does in water. Sound also travels faster in beryllium than it does in steel.

Speed of sound - Wikipedia, the free encyclopedia

Light doesn't need a medium but it will not follow a vacuum the way sound follows air or water or steel.
The best I can do right now, I'll try to look into it further.

[InfiniteNow]

Sound also propogates through materials, being that sound is a disturbance of the medium. So, for example, sound goes through walls, but visible light will not. Further, sound is more able to reflect off of surfaces, whereas light must hit a reflective surface (like a mirror) to show any significant degree of focussed reflection (otherwise, it is absorbed and scattered and the signal is basically lost).

They are two completely different phenomenon, and should not be confused as the same. Case in point, when did you ever hear a particle of sound, or an audible cue that displayed both particle and wave qualities depending on how it was measured?

Apples and oranges.

[Erasmus00]

Originally Posted by InfiniteNow

Case in point, when did you ever hear a particle of sound, or an audible cue that displayed both particle and wave qualities depending on how it was measured?

These are called phonons, light and sound aren't so dissimilar.
-Will

[InfiniteNow]

Originally Posted by Erasmus00

These are called phonons, light and sound aren't so dissimilar.

Well, I'll be darned. Do these only occur in crystalline lattices, or are they measurable in common everyday situtuations like our hallways and bathrooms? Or... are they a probabilistic wave only? I'd welcome any further insight, as you've just taught me something completely new.



EDIT: Okay, I've just ploughed through about 3 articles after googling, and I think I've figured much of it out. It makes a lot of sense, and I can see that I was working from a classical interpretation of sound, whereas that's not always so accurate. Thanks again, Will.


Phonons and the Debye Specific Heat

"The vibrational energies of molecules, e.g., a diatomic molecule, are quantized and treated as quantum harmonic oscillators. Quantum harmonic oscillators have equally spaced energy levels with separation DE = hu. So the oscillators can accept or lose energy only in discrete units of energy hu.

The evidence on the behavior of vibrational energy in periodic solids is that the collective vibrational modes can accept energy only in discrete amounts, and these quanta of energy have been labeled "phonons". Like the photons of electromagnetic energy, they obey Bose-Einstein statistics.

Considering a solid to be a periodic array of mass points, there are constraints on both the minimum and maximum wavelength associated with a vibrational mode."

I've also read that they aren't really actual particles, but more like holes (the absence of an electron) in semiconductors.