# Coherent waves?

1. Jul 15, 2011

### khurram usman

is it necessary for coherent waves to be of same frequency? because everywhere i read its written that they must have a constant phase difference. but cant they have frequencies that are simple numerical multiples of each other?
like 256 512

Last edited by a moderator: Jul 19, 2011
2. Jul 18, 2011

### Claude Bile

Real waves are never perfectly coherent because they possess a finite frequency bandwidth. Over time and distance, the phase difference between the frequency components of a real wave will accumulate until the wave is no longer coherent.

Coherence length is a quantity that is often used to express the degree of coherence of a real wave.

Claude.

3. Jul 18, 2011

### khurram usman

claude can you please explain in a bit layman terms?

Last edited by a moderator: Jul 19, 2011
4. Jul 18, 2011

### Drakkith

Staff Emeritus
He mans that two waves that are measured at, say 500 hz, over time will show that they are very slightly different. One might be 500.00001 hz, while the other might be 499.999997 hz. So over time they will become out of phase with each other and start to interfere destructively.

Last edited by a moderator: Jul 19, 2011
5. Jul 19, 2011

### iiternal

Two waves are always either constructive or distructive, if you can measure them fast enough.
If they do not have the same frequency, or similar, the constructive or distructive patten changes (the changing rate denpending the difference between the frequencies). That's why you cannot see a "coherent".
In real life, no two waves have exactly the same frequency (unless they oringin from a same wave). However, the patten changes very slowly, and you can see it clearly. You say they are coherent.

6. Jul 20, 2011

### Claude Bile

Sort of.

Let's say we try to measure the frequency of a laser beam. We might measure a value of, say 40 THz, however in actuality, there will be lots of frequencies (infinitely many in fact) centred around 40 THz.

Now, these frequencies will lie within a certain range, lets say 8 MHz. So the highest frequency will be 40.00004 THz and the lowest frequency will be 39.9996 THz. Over time (and distance), these frequencies will drift out of phase and the laser beam loses its coherence.

Claude.

7. Jul 20, 2011

### Dotini

Since there are many frequencies involved, would lightning be a good example of an incoherent waveform?

Respectfully,
Steve

8. Jul 20, 2011

### iiternal

The question is not well defined. You can see lightening because it flashes, and you see the light. In the air, the frequency of light can be distinguished by its color. Typically, the flash contains a wide spectra (frequency width), it is almost impossible to see the destructive or constructive pattern.

9. Jul 20, 2011

### Dotini

Sorry, I had meant to include this link, which shows a spectrum of lightning.: http://science.nasa.gov/science-news/science-at-nasa/2001/ast19jan_1/

Respectfully,
Steve