Difference between Coherent and "In Phase" waves?

Question

Does In-phase light mean when waves reach the same point in their cycle at the same time? Or does this just refer to "exactly in phase"?

Does "coherent light" mean "in phase light"?

If coherent means "constant phase difference", surely that is different to "in phase" then?

Im so confused, someone please help.

Answer 1

Phase difference is the fraction of a cycle between waves. For two waves to be in phase, they must have an exact phase difference I.e. pi, 2pi ,3pi etc. ( 180, 360 degrees etc)

Waves are coherent if two sources of waves are in phase as explained above. This means that they if you imagine them together, their peaks and anti peaks will line up.



A phase difference can be any radian e.g. 5/6 pi in which case the sources are not coherent as the waves are not exactly in phase. For phase to be exact, the difference must be a multiple of pi radians or 180 degrees

Answer 2

Any two random waves could momentarily 'be in phase' with each other such that a peak or a valley overlap, interference does occur but is not easily seen.

However coherent waves are identical in amplitude and phase such that they could constructively or destructively interfere with each other when they meet and this is easy to see. Two peaks or two valleys together sum up to a higher intensity whereas a peak with a valley will appear to cancel with no intensity seen.



The overlap of two non coherent beams would be just a random hash and very difficult to see what is happening but will in fact be perceived as an image by our eyes as a tree, a house etc.

In other words normal regular everyday illumination that our eyes detect.





A constant phase difference just means that two waves set out at different times so the amount of mis-match between them never changes.

Answer 3

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