# Speed of light is it really constant?

1. Jul 20, 2011

### deathtical

I was just wondering because as I understand it, there are different wave lengths of light. Also, if I understand it correctly, "light" is both a wave and a particle (photon). Now, I was a SONAR specialist in the Navy and one of the things we had to learn there was that sound waves propagate/travel at different rates based on their wave length/frequency and the amount of power used to generate them. This is how we were able to determine how far away a sound source was and of course if it was a natural or man made sound. So... since light has different frequencies/wave lengths, wouldn't that mean that light has a speed range rather than just one? Or, when science refers to the speed of light, are they just referring to a single light frequency as well? Thanks.

2. Jul 20, 2011

### Ryan_m_b

Staff Emeritus
The speed of light is a constant; wavelength, frequency, amplitude etc do not effect how fast light travels. The comparison with sound waves doesn't stand up as they are very different phenomenon. The speed of light does change with regards to the medium through which it travels, when scientists (or most people) talk about the speed of light they are normally referring to c which is the speed of light in a vacuum.

3. Jul 20, 2011

### deathtical

Ah, ok. I thought the sound example might hold more weight since light does produce a "sound". We cannot hear it without assistance from a machine/computer but it does produce one. Actually, anything that "vibrates" (ie: has a wave length) produces a sound. I just depends on whether you have the ability to perceive it as such. Anyway... thanks.

4. Jul 20, 2011

### Staff: Mentor

....um, sound (and sonar) doesn't work that way either. Distance traveled varies with frequency, but speed does not.

...nor is itthe correct to say light is/has sound. light is not a [mechanical] vibration.

5. Jul 20, 2011

### deathtical

Good catch. I should have said the density of the medium affects the speed and distance that sound travels. Hmm... that doesn't look right either. I defiantly know it affects how far it travels. A sound in water can be heard half way around the world (providing it is in the right propagation layer) where the same sound in the atmosphere may only be heard a few meters. Now I think about it the speed is constant and that's how we determined the distance to the target.

For light having sound. I think I may have confused that with the sound that stars make that can be heard on a radio telescope. All that churning and bubbling

6. Jul 20, 2011

### Drakkith

Staff Emeritus
Yeah, the radio signals can be converted to a sound, but light itself does not make a sound.