# Do I have the right understanding for a wavefront for sound and light?

1. Aug 24, 2014

### jlyu002@ucr.e

When a sound/light source propagates waves, do these waves travel radially(as in 360 degrees in the x y and z coordinate of space, like an electric field from a proton) around the source. For example, in the attachment I sent, is this a correct representation of a helicopter view of the waves being emited?

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2. Aug 24, 2014

### davenn

I suspect you mean radiating out in a sphere ?

Yes they will IF the source is isotropic

can you think of some sources that would radiate sound and or light equally ( or close to) in all directions ?

hint ....
light from a flashlight ( torch) wouldn't
Sound from a speaker wouldn't

have a think about that and come back with some answers and we will see how you got on

Dave

3. Aug 24, 2014

### jlyu002@ucr.e

Ooo I think I am getting it.

For a hypothetical bulb that is floating in space without any light obstruction, then it is radiating out in a sphere.
For a flashlight, the light is not all around because behind the flashlight blocks the light, therefore the light is limited.

I'm scratching my head for the reason for a sound wave, but if the speaker was hanging in space with all sides emitting sounds, then it would radially, but I am not sure for a regular speaker that emits in a certain direction? I think there is a limit to the propagation of the air for example, the I is the speaker and the < is the boundary of the sound wave: I<.

What is the low amplitude for light, does that mean that the electric field is at low amplitude, or the bfield is at the low amplitude. Which do we use?

4. Aug 24, 2014

### davenn

OK consider the sun .... it could be termed an isotropic radiator of light. It is a sphere and will radiate light in all directions
or consider an explosion in the atmosphere or space ( nuclear or chemical doesn't really matter)
sound and light will radiate in all directions

A light bulb (globe) would normally not radiate light in all directions ( tho would be close) because of the shape of the filament construction .... its usually a long construction so the light radiated off the ends of the filament would be low to nil ( this is the same for a dipole radio antenna --- nil off the ends)

I have seen some commercial speakers that come close to a spherical sound emission pattern. but for most the sound is primarily radiated in one direction

with those examples, anything else you can think of ?

Dave

5. Aug 25, 2014

### sophiecentaur

If the speed is the same in all directions (i.e. an isotropic medium) then the peaks of the wave will spread out over a spherical surface. But that doesn't imply that the amplitude has to be the same in all directions - you can have a directive beam.
There is one small caveat and that is that the detailed phase of the wavefront may be different over the sphere, because of the physical extent of the source and the details of how it works. The spherical wavefront may be slightly offset or 'crinkly' as a result but the departure from a true sphere will not be greater than a wavelength or so. The further away you get, the less relevant the small irregularities become.