# Sound & light waves

#### Raparicio

Dear friends,

What are de principal differences between sound and light waves?

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#### flo123

Are they both electomagnetic??

Sound waves are caused by vibrations of particles but can act like emwaves do. for example diffraction patterns, showing up constuctive and destuctive waves by loud spots and quiet spots.

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#### Raparicio

Sound waves moves at the line of movement, and light are perpendicular... any more???

#### flo123

waves

sound waves are parallel to the direction of travel so there longitudinal

light waves are perpendicular to direction of travel, they oscillate at right angles to each other. - Transverse

#### dextercioby

Homework Helper

We call these waves longitudinal (like the sound waves) and transversal (like the em.waves).I'm sure an optics book will cover em waves and their polarization.

I'm sure a continuous media book will cover elastic/mechanical waves.So my advice is to go to the library.

Daniel.

#### Raparicio

dextercioby said:

We call these waves longitudinal (like the sound waves) and transversal (like the em.waves).I'm sure an optics book will cover em waves and their polarization.

I'm sure a continuous media book will cover elastic/mechanical waves.So my advice is to go to the library.

Daniel.
any recomendation???

#### dextercioby

Homework Helper
A good treatment on electromagnetic waves and radiation is given in Born & Wolf's "Principles of Optics",even though ones might claim it's a bit old.

Also,the classical texts of Landau & Lifschitz and Jackson give good insights.

Sound waves are dealt with quite nicely in Landau & Lifschitz "Fluid Mechanics".

Daniel.

Thanks

thks

#### OlderDan

Homework Helper
The longitudinal (sound) vs transverse (light) difference is very important, but another major difference is that has barely been mentioned is that sound waves can only travel through matter, while light waves can travel through empty space as pure energy propegation. If the comparison is between audible sound and visible light, the frequency, speed, and wavelength differences are noteworthy and important for understanding some aspects of why light interacts with matter so differently than sound.

#### Raparicio

are light waves, plane waves? I've seen the example that always shows a electro-magnetic wave, with one 90º in diference to the other. But, really is this way?

#### OlderDan

Homework Helper
Raparicio said:
are light waves, plane waves? I've seen the example that always shows a electro-magnetic wave, with one 90º in diference to the other. But, really is this way?
The term "plane wave" refers to the shape of the wave front propegating through space. By "wave front", we mean a surface of constant phase. This surface is perpendicular to the direction of propegation of the wave. A wave produced by a point source of light will propegate outward in all directions. The wave front will be spherical, and such a wave is called a spherical wave. The energy of the wave is being spread over a larger and larger surface as the wave propegates, so the intensity of the light diminshes with distance, inversely proportianal to the area of a sphere or distance squared.

At distances far away from a point source, over a limted area, the wave front will be indistinguishable from a plane wave front. In theory, a point source at the focus of a parabolic reflector will be reflected as a perfect plane wave. A laser produces a beam that is very nearly a plane wave. For a perfect plane wave in a vacuum, there would be no reduction in intensity as the wave propegates.

The transverse nature of light waves has to do with the electric and magnetic fields that are propegating. These fields are perpenducular to one another at all points in space. I'm thinking the diagram to which you refer is showing this relationship, with the electric field vectors in one plane and the magnetic field vectors in plane at 90 degrees. Such a wave is called a "plane polarized" or "linearly polarized" wave, which is not the same thing as a plane wave. Other polarizations are possible, such as circular polarization or even elliptical polarization. Such waves can still all be plane waves.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html#c1

#### Raparicio

Thanks OlderDan for your time and explanation. Has been very clear for me.

raparicio.

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