The difference of total phase angles of two particles executing simple harmonic motion with respect to the mean position is known as the phase difference. Two vibrating particles are said to be in the same phase, the phase difference between them is an even multiple of π.
ΔΦ = nπ where n = 0, 1, 2, 3, . . . . .
Two vibrating particles are said to be in opposite phase if the phase difference between them is an odd multiple of π.
ΔΦ = (2n + 1) π where n = 0, 1, 2, 3, . . . . .