Coherent wave?

Question

"two sources of waves are coherent if their PHASE DIFFERENCE are constant"

is the statemeny above true?

i am wondering whether coherent wave needed to be in phase or not. recently studying about Young's double slit experiment.

How if in the case where phase difference is pie/2 ? How does it form constructive interference at the centrel bright fringe while the peak doesn't meet wif peak?

Answer 1

It is the phase difference emerging from the double slits that gives rise to constructive or destructive interference! For a mono-chromatic coherent light source - constructive interference occurs when the waves have a phase difference of 0 or  2π radians. However, destructive interference occurs when the light waves have a phase difference of π radians. Any intermediate phase difference will result in partial addition or reduction of wave amplitudes.

Finally, Wikipedia adds, 'Coherence (from Latin cohaerere = to be connected) describes in physics a property of waves, that enables stationary (i.e. temporally and spatially constant) interference. More generally, coherence describes all correlation properties between physical quantities of a wave.

When interfering, waves add constructively or subtract destructively, depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase, which also implies that they have the same frequency. The degree of coherence is measured by the interference visibility, a measure of how perfectly the waves can cancel due to destructive interference.'

Thus, your initial statement is correct!