Two particles executes S.H.M. of same amplitude and frequency along the same straight line. They pass one another when going in opposite directions, and each time their displacement is half of their amplitude. The phase difference between them is

y=asin⁡ωt+ϕ0. According to the question y=a2⇒a2=asin⁡ωt+ϕ0⇒ωt+ϕ0=ϕ=π6 or 5π6Physical meaning of ϕ=π6 : particle is at point p and it is going towards Physical meaning of ϕ=5π6: particle is at point P and it is going towards OSo phase difference, Δϕ=5π6−π6=2π3=120∘