What does coherent exactly mean?

[primarygun]

What does coherent exactly mean? What does same phase difference refer to?
Why these two factors must be present so as to create interference?
I'd think if a wave which has a half wavelength to another, same speed.
the phenomenon can be observed clearly too.

 

[Astronuc]

Good discussion on the significance of coherence, and coherent light, which is the basis of a laser.

http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/qualig.html#c4

Coherent implies that light photons (i.e. the EM waves) are in phase, i.e. they constructively interfere.

 

[marlon]

https://www.physicsforums.com/journal.php?s=&action=view&journalid=13790&perpage=10&page=9

Check out the 'for microelectrinics and nanotechnology lovers' entry


There are two types of coherency : spatial and time-like coherence. For example let's look at a wave that consists out of two subwaves. If you would look at some point on each subwave, you will see that the distances between those two points (one point on each subwave) does not alter when the wave propagates through space and time. One subwave does not run away from the other subwave, they stay together. This kind of wave is very ordened and no subwave will scatter off.

marlon

 

[primarygun]

Thank you.
So water wave is really different from light wave as a light wave contain many incoherent small wave but water wave does not contain?