Q.Answer in one word/sentence: Give the relation between phase difference and path difference.

The phase of a vibrating particle is its complete instantaneous state of oscillation -- fixed by knowing its displacement, the direction its velocity currently points, and which oscillation cycle it is currently in. Two particles are 'in phase' when both their displacement and their velocity direction match exactly. Phase is most conveniently expressed as an ANGLE: a particle at the very leading edge of a disturbance is assigned phase 0°; a quarter-wavelength behind it, at its first positive maximum, phase is 90° ($\pi^c/2$); a half-wavelength behind, phase is 180° ($\pi^c$); a full wavelength behind, phase is 360° ($2\pi^c$) -- and so on, with each further wavelength of separation adding another full $2\pi^c$. …